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):
td><td class='form'><form class='right' method='get' action='/guix/log/tests/uuid.scm'>
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<option value='grep'>log msg</option>
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</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=ad7749a1273ccd2556acd4d24388956d6962a723&amp;showmsg=1'>root</a>/<a href='/guix/log/tests?id=ad7749a1273ccd2556acd4d24388956d6962a723&amp;showmsg=1'>tests</a>/<a href='/guix/log/tests/uuid.scm?id=ad7749a1273ccd2556acd4d24388956d6962a723&amp;showmsg=1'>uuid.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/uuid.scm?id=ad7749a1273ccd2556acd4d24388956d6962a723'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2019-05-07 12:18:44 +0200'>2019-05-07</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=6c5790a2faf8ffc401e43b13425a707394e40874'>uuid: 'fat-uuid-&gt;string' preserves leading zeros.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Fixes &lt;https://bugs.gnu.org/35582&gt;.
Reported by sirgazil &lt;sirgazil@zoho.com&gt;.

Previously, leading zeros would be removed, leading to an "invalid"
UUID:

  (uuid-&gt;string (uuid "00CA-050E" 'fat32))
  ⇒ "CA-50E"
  (string-&gt;uuid "CA-50E" 'fat32)
  ⇒ #f

* gnu/system/uuid.scm (fat-uuid-&gt;string): Pad digits with zeros.
* tests/uuid.scm ("uuid, FAT32, leading zeros preserved"): New test.


</td></tr>
<tr class='logheader'><td><span title='2018-05-19 00:14:52 +0200'>2018-05-19</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=263c9941a1e523b360ca9f42d1ed6b11e6e6e285'>uuid: 'uuid' returns #f when 'string-&gt;uuid' returns #f.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/system/uuid.scm (uuid): When STR is not a literal, return #f when
'string-&gt;uuid' returns #f.
* tests/uuid.scm ("uuid, dynamic value"): New test.


</td></tr>
<tr class='logheader'><td><span title='2017-10-06 14:16:23 +0200'>2017-10-06</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=0b12324284f3fbf378d363070f91b794058ae570'>uuid: Fix typo in 'uuid=?' test.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* tests/uuid.scm ("uuid=?"): Change to 'test-assert'.


</td></tr>
<tr class='logheader'><td><span title='2017-10-05 12:09:17 +0200'>2017-10-05</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=aed1f1b049f6e7ef9f2f2db58bdca6cd3effe94b'>uuid: Add 'uuid=?' and use it.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/system/uuid.scm (uuid=?): New procedure.
* tests/uuid.scm ("uuid=?"): New test.
* gnu/build/file-systems.scm (partition-uuid-predicate)
(luks-partition-uuid-predicate): Use it instead of 'bytevector=?'.


</td></tr>
<tr class='logheader'><td><span title='2017-09-22 18:40:57 +0200'>2017-09-22</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=8a7d81a5e23c4d59fbabf2550db32d4ba5572e4b'>uuid: Add a parser for FAT32 UUIDs.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/system/uuid.scm (%fat32-uuid-rx): New variable.
(string-&gt;fat32-uuid): New procedure.
(%uuid-parsers): Add it.
* tests/uuid.scm ("uuid, FAT32, format preserved"): New test.


</td></tr>
<tr class='logheader'><td><span title='2017-09-14 00:10:13 +0200'>2017-09-14</span></td><td class='logsubject'><a href='/guix/commit/tests/uuid.scm?id=8f3b63b5d73c0b49139c8a20b7576732abfdeeb8'>uuid: Move tests to 'tests/uuid.scm'.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* tests/file-systems.scm ("uuid-&gt;string", "string-&gt;uuid")
("uuid", "uuid, syntax error"): Move to...
* tests/uuid.scm: ... here.  New file.
("uuid, ISO-9660, format preserved"): New test.


</td></tr>