Disable systemd-run if the command is not found.

diff -Naur xpra-4.0.1/xpra/scripts/main.py xpra-4.0.1.patched/xpra/scripts/main.py
--- xpra-4.0.1/xpra/scripts/main.py	2020-05-17 18:12:15.000000000 +0200
+++ xpra-4.0.1.patched/xpra/scripts/main.py	2020-06-01 12:12:18.500257507 +0200
@@ -312,16 +312,18 @@
     if not is_systemd_pid1():
         return False
     #test it:
-    cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
-    proc = Popen(cmd, stdin=None, stdout=None, stderr=None, shell=False)
-    r = pollwait(proc, timeout=1)
-    if r is None:
-        try:
-            proc.terminate()
-        except Exception:
-            pass
-    return r==0
-
+    try:
+        cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
+        proc = Popen(cmd, stdin=None, stdout=None, stderr=None, shell=False)
+        r = pollwait(proc, timeout=1)
+        if r is None:
+            try:
+                proc.terminate()
+            except Exception:
+                pass
+        return r==0
+    except FileNotFoundError:
+        return False
 
 def run_mode(script_file, error_cb, options, args, mode, defaults):
     #configure default logging handler:
><a href='/guix/tree/tests/print.scm?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>tree</a><a href='/guix/commit/tests/print.scm?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>commit</a><a href='/guix/diff/tests/print.scm?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/print.scm'>
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<div class='path'>path: <a href='/guix/log/?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>root</a>/<a href='/guix/log/tests?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>tests</a>/<a href='/guix/log/tests/print.scm?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69'>print.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/print.scm?id=94ca5b4357af8f8921f0cb0873a7cf316f13aa69&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-09-29 22:59:48 +0200'>2022-09-29</span></td><td><a href='/guix/commit/tests/print.scm?id=8b2119a5e4793cad1df1070d488a54ceea420840'>import: print: Quasiquote properties</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (package-&gt;code): Quasiquote properties.
* tests/print.scm (pkg-with-properties, pkg-with-properties-source):
New variables.
("package with properties"): New test

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>itd</td></tr>
<tr><td><span title='2021-11-17 23:43:14 +0100'>2021-11-17</span></td><td><a href='/guix/commit/tests/print.scm?id=3a317f7476f8c6012e166ff9f340f861938721c9'>Merge branch 'master' into core-updates-frozen</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-11 00:14:31 +0100'>2021-11-11</span></td><td><a href='/guix/commit/tests/print.scm?id=3756ce32674139376bcf11dac96bc562582088f7'>import: print: Replace packages and origins in 'arguments'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (package-&gt;code)[variable-reference]
[object-&gt;code]: New procedures.
[package-lists-&gt;code]: Rewrite in terms of 'object-&gt;code'.
Pass the 'arguments' field through 'object-&gt;code'.
* tests/print.scm (pkg-with-arguments, pkg-with-arguments-source): New
variables.
("package with arguments"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-11 00:14:31 +0100'>2021-11-11</span></td><td><a href='/guix/commit/tests/print.scm?id=b2ed40c29f578d46d42cb1c5e99bd797cea3aba0'>import: print: Handle patches that are origins.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (package-&gt;code)[source-&gt;code]: Handle patches
that are origins.
* tests/print.scm (pkg-with-origin-input): Add 'patches' field.
(pkg-with-origin-patch, pkg-with-origin-patch-source): New variables.
("package with origin patch"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-11 00:14:31 +0100'>2021-11-11</span></td><td><a href='/guix/commit/tests/print.scm?id=b3240ae846cb1ace2322a68eca3497f11d0be6f1'>import: print: Correctly handle URI lists.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (package-&gt;code)[factorized-uri-code]: New
procedure.
[source-&gt;code]: Use it, and factorize URI when it's a list.
* tests/print.scm (pkg-with-origin-input): Check origin URI to a list.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-11-11 00:14:31 +0100'>2021-11-11</span></td><td><a href='/guix/commit/tests/print.scm?id=04d929570ad816793d7e0024a11314124ce87f98'>import: print: Properly render packages with origins as inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (package-&gt;code)[source-&gt;code]: Check whether
VERSION is true before calling 'factorize-uri'.
[package-lists-&gt;code]: Add clause for inputs that are origins.
* tests/print.scm (pkg-with-origin-input, pkg-with-origin-input-source):
New variables.
("package with origin input"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-07-11 00:49:16 +0200'>2021-07-11</span></td><td><a href='/guix/commit/tests/print.scm?id=ff992fcfaf8455910b4bb5e02861fe9ae3dfd974'>import: print: Emit new-style package inputs when possible.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/import/print.scm (redundant-input-labels?): New procedure.
(package-&gt;code)[package-lists-&gt;code]: Rename to...
[inputs-&gt;code]: ... this.  When 'redundant-input-labels?' returns true,
emit label-less inputs.  Adjust callers to new name.
* tests/print.scm (pkg-with-inputs): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>