Disable lockdown for external forks by default

GitHub Actions can be used in external forks, and this
workflow breaks all pull-requests for those who use both
GitHub Pull Requests and GitHub Actions.

Reviewed By: tstellar

Differential Revision: https://reviews.llvm.org/D112801

.github/workflows/repo-lockdown.yml

@@ -9,6 +9,7 @@ permissions:
 jobs:
   action:
     runs-on: ubuntu-latest
+    if: github.repository == 'llvm/llvm-project'
     steps:
       - uses: dessant/repo-lockdown@v2
         with: