workflows: Don't try and run llvm-bugs in forks

Most forks won't have an LLVM_BUGS_KEY secret, so Mailgun will error out
with a missing key, and if forks do happen to have such a secret then we
still don't want to be getting emails to LLVM's mailing lists for
downstream forks.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D146235