makebuildserver: fix join() syntax error, it needs a list/tuple
makebuildserver: fix join() syntax error, it needs a list/tuple

If only there was a way to test this without taking hours to run...

fixes 964ef996a01ceac8d6735668f2c9fd9119512346

fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver