apt-key: ignore gpg1 already imported secret key import failure

apt-key: ignore gpg1 already imported secret key import failure

On Travis and co the default gpg implementation is gpg1 which for some
reason fails if a secret key which was already imported is imported
again. We would prefer it to be a NOP like gpg2 handles it so we crudely
check the error message. apt-key usually doesn't deal with secret keys –
it only learned to do it for manual testing and the integration
framework usage, so no public interface is effected.

Triggered-By: 4ce2f35248123ff2366c8c365ad6a94945578d66
Gbp-Dch: Ignore