Disable systemd-run if the command is not found.

diff -ru xpra-4.2~/xpra/scripts/main.py xpra-4.2/xpra/scripts/main.py
--- xpra-4.2~/xpra/scripts/main.py	2021-06-06 08:51:13.756815842 -0700
+++ xpra-4.2/xpra/scripts/main.py	2021-06-06 16:07:13.371024486 -0700
@@ -331,23 +331,26 @@
     if not is_systemd_pid1():
         return False    # pragma: no cover
     #test it:
-    cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
-    proc = Popen(cmd, stdout=PIPE, stderr=PIPE, shell=False)
     try:
-        proc.communicate(timeout=2)
-        r = proc.returncode
-    except TimeoutExpired:  # pragma: no cover
-        r = None
-    if r is None:
+        cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
+        proc = Popen(cmd, stdout=PIPE, stderr=PIPE, shell=False)
         try:
-            proc.terminate()
-        except Exception:
-            pass
-        try:
-            proc.communicate(timeout=1)
+            proc.communicate(timeout=2)
+            r = proc.returncode
         except TimeoutExpired:  # pragma: no cover
             r = None
-    return r==0
+        if r is None:
+            try:
+                proc.terminate()
+            except Exception:
+                pass
+            try:
+                proc.communicate(timeout=1)
+            except TimeoutExpired:  # pragma: no cover
+                r = None
+        return r==0
+    except FileNotFoundError:
+        return False
 
 
 def run_mode(script_file, error_cb, options, args, mode, defaults):
d3887303e260aba433dd1ef'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/datastructures.scm'>
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<div class='path'>path: <a href='/guix/log/?id=0726d4ea7cb882dd1d3887303e260aba433dd1ef'>root</a>/<a href='/guix/log/gnu?id=0726d4ea7cb882dd1d3887303e260aba433dd1ef'>gnu</a>/<a href='/guix/log/gnu/packages?id=0726d4ea7cb882dd1d3887303e260aba433dd1ef'>packages</a>/<a href='/guix/log/gnu/packages/datastructures.scm?id=0726d4ea7cb882dd1d3887303e260aba433dd1ef'>datastructures.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/gnu/packages/datastructures.scm?id=0726d4ea7cb882dd1d3887303e260aba433dd1ef&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-06-15 10:34:22 +0200'>2017-06-15</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=a3f8aae4a159baf2de10c1f8860d7128c89fbb0c'>gnu: liburcu: Update to 0.10.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/databases.scm (liburcu): Update to 0.10.0.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-04-17 02:04:08 +0200'>2017-04-17</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=d1e4ad1b02b2ea888557d3272c50b676faf18ca4'>gnu: Fix typos in descriptions.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/admin.scm (di)[description]: Likewise.
* gnu/packages/bioinformatics.scm (r-annotate)[description]: Likewise.
* gnu/packages/datastructures.scm (sparsehash)[description]: Likewise.
* gnu/packages/dns.scm (knot)[description]: Likewise.
* gnu/packages/emacs.scm (emacs-idle-highlight)[synopsis, description]:
Likewise.
* gnu/packages/gnome.scm (libpeas)[description]: Likewise.
* gnu/packages/gtk.scm (python2-pygtk)[description]: Likewise.
* gnu/packages/kde-frameworks.scm (kactivities)[description]: Fix typo.
* gnu/packages/libevent.scm (perl-anyevent)[description]: Likewise.
* gnu/packages/machine-learning.scm (ghmm)[description]: Likewise.
* gnu/packages/mail.scm (mlmmj)[description]: Likewise.
* gnu/packages/maths.scm (vc)[description]: Likewise.
* gnu/packages/music.scm (gx-super-fuzz-lv2)[description]: Likewise.
* gnu/packages/networking.scm (nload)[description]: Likewise.
* gnu/packages/python.scm (python-execnet)[description]: Likewise.
* gnu/packages/terminals.scm (tilda)[description]: Likewise.
* gnu/packages/python.scm (python-execnet, python-tables)
(python2-coverage-test-runner, python2-rope)[description]: Likewise.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-03-17 02:32:00 +0100'>2017-03-17</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=754667f0eb44491370a2ec4fa0b4705cfcf6d237'>gnu: Add liburcu.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (liburcu): New variable.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-07-19 23:03:30 +0200'>2016-07-19</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=592ccdd38850d581f669f834f051ee8f92d7ee31'>gnu: Add ssdeep.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (ssdeep): New variable.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2016-03-01 10:30:04 +0100'>2016-03-01</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=b4464d384c7a7b500e611ec257cf092cdc47d977'>gnu: Add gdsl.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (gdsl): New variable.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2015-12-11 15:59:20 +0100'>2015-12-11</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=f7f55db8f6d049225898d4ebcd7bc1d53623f32d'>gnu: Move sparsehash to datastructures module.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/crypto.scm (sparsehash): Move variable to...
* gnu/packages/datastructures.scm: ...new file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Ricardo Wurmus</td></tr>