MNT Permission for PRs for GH token in stale.yml (#3112)

Continuation of #3102.

The equivalent PR in
PEFT (huggingface/peft#2064) was successful to
restore stale bot function to PRs as well. Hence also making the same
change for accelerate.

.github/workflows/stale.yml

@@ -12,6 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       issues: write
+      pull-requests: write
     env:
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps: