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):
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<div class='path'>path: <a href='/guix/log/?id=d058b5104ac9b626155062d2696b0b7015023a7b'>root</a>/<a href='/guix/log/gnu?id=d058b5104ac9b626155062d2696b0b7015023a7b'>gnu</a>/<a href='/guix/log/gnu/tests?id=d058b5104ac9b626155062d2696b0b7015023a7b'>tests</a>/<a href='/guix/log/gnu/tests/security-token.scm?id=d058b5104ac9b626155062d2696b0b7015023a7b'>security-token.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/tests/security-token.scm?id=d058b5104ac9b626155062d2696b0b7015023a7b&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-05-06 17:12:45 +0200'>2023-05-06</span></td><td><a href='/guix/commit/gnu/tests/security-token.scm?id=f51888272558d98cf5c196b93fb6c499056fbf6c'>tests: Add missing module imports for marionette-evaluated code.</a><span class='msg-avail'>...<span class='msg-tooltip'>This missing imports became apparent with commit
a09c7da8f8d8e732f969cf0a09aaa78f87032ab1, which runs the marionette
service in a fresh Guile process with fewer imports.

* gnu/tests/databases.scm (run-postgresql-test, run-timescaledb-test)
(run-mysql-test): Add missing module imports for code passed to
'marionette-eval'.
* gnu/tests/docker.scm (run-docker-test, run-docker-system-test): Likewise.
* gnu/tests/mail.scm (run-dovecot-test, run-getmail-test): Likewise.
* gnu/tests/monitoring.scm (run-zabbix-server-test): Likewise.
* gnu/tests/pam.scm (run-test-pam-limits): Likewise.
* gnu/tests/reconfigure.scm (run-switch-to-system-test)
(run-install-bootloader-test): Likewise.
* gnu/tests/security-token.scm (run-pcscd-test): Likewise.
* gnu/tests/install.scm (gui-test-program): Likewise.
* gnu/tests/telephony.scm (run-jami-test): Add modules to
the #:imported-modules argument of 'marionette-operating-system'.
[test]: Remove them from 'with-imported-modules'; remove
'with-extensions'.  Add "d-bus tooling loaded" test to set up %load-path
and %load-compiled-path so the marionette process can find
guile-ac-d-bus and guile-packrat.
</span></span></td><td>Ludovic Courtès</td></tr>