use only one --keyring in gpg interactions

We were down to at most two keyrings before, but gnupg upstream thinks
about dropping support for multiple keyrings in the longrun, so with a
single keyring we hope to be future proof – and 'apt-key adv' isn't a
problem anymore as every change to the keys is merged back, so we have
now the same behavior as before, but support an unlimited amount of
trusted.gpg.d keyrings.