You should not post on Arch forum for issues with Manjaro, you’ll probably receive the adequate response there in a moment 
PS: you didn’t even search, I typed manjaro-keyring in the forum search, and found the thread quickly (it is often suggested in most of the threads in results): [HowTo] Work around gpg verification issue on left behind systems (same as above). Same if you search the actual error invalid or corrupted package.
