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: