always use our own trustdb.gpg in apt-key

always use our own trustdb.gpg in apt-key

APT doesn't care for the trustdb.gpg, but gnupg requires one even for
the simplest commands, so we either use the one root has available in
/etc or if we don't have access to it (as only root can read that file)
we create a temporary directory to store a trustdb.gpg in it.

We can't create just a temporary file as gpg requires the given
trustdb.gpg file to be valid (if it exists), so we would have to remove
the file before calling gnupg which would allow mktemp (and co) to hand
exactly this filename out to another program (unlikely, but still).