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):
d' value='a5481f7cef8e710428b334fbe9c4fbcb1ad0a873'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=a5481f7cef8e710428b334fbe9c4fbcb1ad0a873'>root</a>/<a href='/guix/log/ROADMAP?id=a5481f7cef8e710428b334fbe9c4fbcb1ad0a873'>ROADMAP</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/ROADMAP?id=a5481f7cef8e710428b334fbe9c4fbcb1ad0a873&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-05-20 01:58:04 +0200'>2021-05-20</span></td><td><a href='/guix/commit/ROADMAP?id=c0087d33705f53cc3619a1a43a33602aaecc3a8b'>The #guix channel is hosted by Libera Chat.</a><span class='msg-avail'>...<span class='msg-tooltip'>* README (Contact): Update network name.
* ROADMAP: Likewise.
* doc/contributing.texi (Contributing): Likewise.
* doc/guix.texi (After System Installation): Likewise.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
3d833db0b'>nls: Update.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-23 00:06:58 +0100'>2020-03-23</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=86603d8660e42a3b5d8b407e456a98429943bc7e'>nls: Update.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-14 09:01:29 +0000'>2020-02-14</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=57718bdf6823aef291d114a457c42c984ac42791'>Revert "nls: Update 'es' translation of the manual."</a><span class='msg-avail'>...<span class='msg-tooltip'>I believe this change breaks guix pull:

  ./guix.es.texi:20971: @samp missing closing brace

This reverts commit d156e3fbcd6fc61a39d1d32622b90b0f8f741729.
</span></span></td><td>Christopher Baines</td></tr>
<tr><td><span title='2020-02-14 04:45:53 +0100'>2020-02-14</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=d156e3fbcd6fc61a39d1d32622b90b0f8f741729'>nls: Update 'es' translation of the manual.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2019-08-31 21:29:46 +0200'>2019-08-31</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=b212fef4b076faccdd36335c728fd71321778716'>nls: Update 'es' translation of the manual.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2019-05-13 10:57:11 +0200'>2019-05-13</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=8548cf6e95917f7b56042d2d275a08e91eedf31a'>nls: Update es translation.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-05-01 11:33:32 +0200'>2019-05-01</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=a3b32d57d6677fded92dbd3f4ecafbf9aceeb855'>nls: Fix cross-reference syntax error in 'guix-manual.es.po'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* po/doc/guix-manual.es.po: Replace "@pref" with "@pxref".
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-05-01 10:16:58 +0200'>2019-05-01</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=d3619b296fd1db6441e09bb51b55a4264b9fd0ed'>nls: Update manual translations for de, es, fr, and zh_CN.</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-25 18:55:12 +0200'>2019-04-25</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=57e2b24dc03e44e4e379436e63d29231f1f9d28c'>nls: Update 'es' translation.</a></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2019-04-23 23:56:16 +0200'>2019-04-23</span></td><td><a href='/guix/commit/po/doc/guix-manual.es.po?id=6227e1b7477b71f31ea3bf8a74c32fe7238dcece'>doc: Add Spanish translation.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/local.mk (info_TEXINFOS): Add guix.es.texi.
(TRANSLATED_INFO): Add guix.es.texi and contributing.es.texi.
* po/doc/guix-manual.es.po: New file.
* po/doc/local.mk: Add guix-manual.es.po.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Miguel Ángel Arruga Vivas</td></tr>