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

(cherry picked from commit 2be973e)

.github/workflows/llvm-bugs.yml

@@ -8,6 +8,7 @@ on:
 jobs:
   auto-subscribe:
     runs-on: ubuntu-latest
+    if: github.repository == 'llvm/llvm-project'
     steps:
       - uses: actions/setup-node@v2
         with: