buildserver: only check cache permissions when using libvirt
buildserver: only check cache permissions when using libvirt

VirtualBox runs as the same user as `fdroid`, so the cache does not need to
be accessible by the world.  On libvirt, libvirtd runs the VMs as its own
user, so in that case, the cache dirs must have permissions to let that
user access them.

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