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

Continueing on the track of dropping privileges in all methods, lets
drop it in copy, too, as the reasoning for it is very similar to file
and the interaction between the too quiet interesting as copy kinda
surfed as a fallback for file not being able to read the file. Both now
show a better error message as well as it was previously claiming to
have a hashsum mismatch, given that it couldn't read the file.

Git-Dch: Ignore

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