[macOS signing] Verify signature after signing
This is not tested, but the commands are taken from Craft, so it should work.
The idea is to make the signing fail if the signature validation failed.
We had several cases recently where the signing worked, but still the signature was invalid. Craft noticed that and wrote it to the log, but we can't force this on the Craft level for multiple reasons (one of them being, that Craft does not know if the branch was cleared for signing and hence would always fail if not).