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):
/guix/log/gnu/artwork.scm'>
<input type='hidden' name='id' value='cf9c76223f028dce92bfe5b242a4faeba73f9d02'/><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=cf9c76223f028dce92bfe5b242a4faeba73f9d02'>root</a>/<a href='/guix/log/gnu?id=cf9c76223f028dce92bfe5b242a4faeba73f9d02'>gnu</a>/<a href='/guix/log/gnu/artwork.scm?id=cf9c76223f028dce92bfe5b242a4faeba73f9d02'>artwork.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/artwork.scm?id=cf9c76223f028dce92bfe5b242a4faeba73f9d02&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-11 11:55:19 -0400'>2022-08-11</span></td><td><a href='/guix/commit/gnu/artwork.scm?id=2b579fd4f453bb1a3592eb8e634ea1dff64efab6'>gnu: artwork: Update repository to latest commit.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/artwork.scm (%artwork-repository): Update to commit
4c7f2ce6428a63e202cd2a9474a06f68a946934d.
</span></span></td><td>Maxim Cournoyer</td></tr>