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:
/libstore/pathlocks.cc'>log</a><a href='/guix/tree/nix/libstore/pathlocks.cc?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>tree</a><a href='/guix/commit/nix/libstore/pathlocks.cc?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>commit</a><a href='/guix/diff/nix/libstore/pathlocks.cc?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/nix/libstore/pathlocks.cc'>
<input type='hidden' name='id' value='3bacd3c76a58ebe70f98be654f09cbd4166093ab'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
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</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=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>root</a>/<a href='/guix/log/nix?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>nix</a>/<a href='/guix/log/nix/libstore?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>libstore</a>/<a href='/guix/log/nix/libstore/pathlocks.cc?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab'>pathlocks.cc</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/nix/libstore/pathlocks.cc?id=3bacd3c76a58ebe70f98be654f09cbd4166093ab&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>