Revised the way how the temporary local apt repository is created and removed.
- Fixed a bug when an own signing key was used to sign the local apt repository. No longer copying the whole pubring.gpg to /etc/apt/trusted.gpg.d/, only copying the specific signing key.
- Fixed a failure bug when WHONIX_BUILD_APT_CODENAME was not set to local (now also works for stable, testers and developers).
- Reduced code duplication.