makebuildserver: explicitly set debian_mirror by default
makebuildserver: explicitly set debian_mirror by default

There needs to be a default, otherwise it crashes.  I should have included
this in 8d66742dd7feec77aa30382bfb38da83f2b3e5a4

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