Don't run issue reviver on forks.

Add a conditional to avoid running the issue reviver on forks. It will
always cause errors since bug references in the source code don't match
issue IDs in forked repos.

PiperOrigin-RevId: 321042060

.github/workflows/issue_reviver.yml

@@ -4,11 +4,13 @@ on:
     - cron: '0 0 * * *'
 
 jobs:
-  label:
+  issue_reviver:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      if: github.repository == "google/gvisor"
     - run: make run TARGETS="//tools/issue_reviver"
+      if: github.repository == "google/gvisor"
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         GITHUB_REPOSITORY: ${{ github.repository }}