run: Support PulseAudio socket path without unix: prefix

https://www.freedesktop.org/wiki/Software/PulseAudio/Documentation/User/ServerStrings/
says:

> If the string starts with / or unix: the remaining address string is taken as
> UNIX socket name.

but previously the string was only taken to be a UNIX socket name if it began
with "unix:".

This is an issue in practice with WSLg <https://github.com/microsoft/wslg>
which sets $PULSE_SERVER to "/mnt/wslg/PulseServer", without a "unix:" prefix.

common/flatpak-run.c

@@ -591,8 +591,11 @@ flatpak_run_parse_pulse_server (const char *value)
             continue;
           server = closing + 1;
         }
+
       if (g_str_has_prefix (server, "unix:"))
         return g_strdup (server + 5);
+      if (server[0] == '/')
+        return g_strdup (server);
     }
 
   return NULL;