fix(ck): allocate unique Xvfb displays (#5074)

lgsm/config-default/config-lgsm/ckserver/_default.cfg

@@ -181,7 +181,7 @@ glibc="2.15"
 ## Game Server Directories
 systemdir="${serverfiles}"
 executabledir="${serverfiles}"
-preexecutable="xvfb-run"
+preexecutable="xvfb-run -a"
 executable="./CoreKeeperServer"
 servercfgdir="${systemdir}/gamedata/${selfname}"
 servercfg="ServerConfig.json"