Disable systemd-run if the command is not found.

diff -Naur xpra-4.0.1/xpra/scripts/main.py xpra-4.0.1.patched/xpra/scripts/main.py
--- xpra-4.0.1/xpra/scripts/main.py	2020-05-17 18:12:15.000000000 +0200
+++ xpra-4.0.1.patched/xpra/scripts/main.py	2020-06-01 12:12:18.500257507 +0200
@@ -312,16 +312,18 @@
     if not is_systemd_pid1():
         return False
     #test it:
-    cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
-    proc = Popen(cmd, stdin=None, stdout=None, stderr=None, shell=False)
-    r = pollwait(proc, timeout=1)
-    if r is None:
-        try:
-            proc.terminate()
-        except Exception:
-            pass
-    return r==0
-
+    try:
+        cmd = ["systemd-run", "--quiet", "--user", "--scope", "--", "true"]
+        proc = Popen(cmd, stdin=None, stdout=None, stderr=None, shell=False)
+        r = pollwait(proc, timeout=1)
+        if r is None:
+            try:
+                proc.terminate()
+            except Exception:
+                pass
+        return r==0
+    except FileNotFoundError:
+        return False
 
 def run_mode(script_file, error_cb, options, args, mode, defaults):
     #configure default logging handler:
/openrc/guix-daemon.in'>log</a><a href='/guix/tree/etc/openrc/guix-daemon.in?id=6a23664cd869697eed8939312e986c862dd40b6d'>tree</a><a href='/guix/commit/etc/openrc/guix-daemon.in?id=6a23664cd869697eed8939312e986c862dd40b6d'>commit</a><a href='/guix/diff/etc/openrc/guix-daemon.in?id=6a23664cd869697eed8939312e986c862dd40b6d'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/openrc/guix-daemon.in'>
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<div class='path'>path: <a href='/guix/log/?id=6a23664cd869697eed8939312e986c862dd40b6d'>root</a>/<a href='/guix/log/etc?id=6a23664cd869697eed8939312e986c862dd40b6d'>etc</a>/<a href='/guix/log/etc/openrc?id=6a23664cd869697eed8939312e986c862dd40b6d'>openrc</a>/<a href='/guix/log/etc/openrc/guix-daemon.in?id=6a23664cd869697eed8939312e986c862dd40b6d'>guix-daemon.in</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/etc/openrc/guix-daemon.in?id=6a23664cd869697eed8939312e986c862dd40b6d&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-07-07 23:54:20 -0400'>2021-07-07</span></td><td><a href='/guix/commit/etc/openrc/guix-daemon.in?id=4cbe0127a668cbcc64087c5d311308b59d39b104'>guix-install.sh: Prompt for configuring substitutes discovery.</a><span class='msg-avail'>...<span class='msg-tooltip'>Also ensure prompt_yes_no always print the message with a trailing space,
which is more pleasing to the eye.

* etc/guix-daemon.conf.in &lt;--discover=no&gt;: New guix-daemon option.
* etc/guix-daemon.service.in: Likewise.
* etc/init.d/guix-daemon.in: Likewise.
* etc/openrc/guix-daemon.in: Likewise.
* etc/guix-install.sh (configure_substitute_discovery): New procedure.
(sys_enable_guix_daemon): Ask the user whether automatic substitute discovery
should be enabled.  Set the '--discover' argument accordingly.
(prompt_yes_no): Add a trailing space to the message.
(sys_authorize_build_farms): Remove trailing space from the message argument.
* NEWS (Distribution): Add news.
</span></span></td><td>Maxim Cournoyer</td></tr>