[gpg] Set GPG home directory when reading signature IDs. Fixes JB#44305
GPG will not start unless it finds or can create ~/.gnupg for its conf and keyrings, and PackageKit runs without a valid home. For every other key operation it sets the home to temporary directories containing general or trusted keyrings, but reading a signature ID from a file shouldn't need one so it's not done right now. Also, zypp shouldn't ever touch the user's keyrings, as it should operate exclusively with keys from the RPM db.
This patch sets the keyring to the general keyring home for this call. It fixes PackageKit, and prevents a permanent ~/.gnupg from being created.