Fix permissions for scan orphaned PRs workflow (#829)

Noticed this problem on a private repo. Didn't notice this problem
before because platform-test is a public repo so it didn't need explicit
permissions to access the pull requests of the public repo.

.github/workflows/scan-orphaned-pr-environments.yml

@@ -38,6 +38,7 @@ jobs:
     permissions:
       contents: read
       id-token: write
+      pull-requests: read
 
     steps:
       - uses: actions/checkout@v4