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

[jrtc27]

Summary

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.

[tstellar]

LGTM.

.github/workflows/llvm-bugs.yml

 name: LLVM Bugs notifier
 
 permissions:
   contents: read
   issues: read
 
 on:
   issues:
     types:
       - opened
 
 jobs:
   auto-subscribe:
     runs-on: ubuntu-latest
+    if: github.repository == 'llvm/llvm-project'
     steps:
       - uses: actions/setup-node@v3
         with:
           node-version: 18
           check-latest: true
       - run: npm install mailgun.js form-data
       - name: Send notification
         uses: actions/github-script@v6