drop privileges in file:// method as we do for decompressors

drop privileges in file:// method as we do for decompressors

We drop it in decompressors, which are the natural next step, so if an
archive is used which isn't worldreadable (= not accessible by _apt) it
doesn't work anyway, so we just fail a bit earlier now and avoid all the
bad things which can happen over file (which could very well still be a
network resourc via NFS mounts or similar stuff, so hardly as safe as
the name might suggest at first).