Add missing GH_TOKEN environment variable

It is needed for using the `gh` CLI commands.

.github/workflows/post-merge.yml

@@ -17,14 +17,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Perform analysis and send PR
+        env:
+          GH_TOKEN: ${{ github.token }}
         run: |
           # Get closest bors merge commit
           PARENT_COMMIT=`git rev-list --author='bors <bors@rust-lang.org>' -n1 --first-parent HEAD^1`
+          echo "Parent: ${PARENT_COMMIT}"
 
           # Find PR for the current commit
           HEAD_PR=`gh pr list --search "${{ github.sha }}" --state merged --json number --jq '.[0].number'`
-
-          echo "Parent: ${PARENT_COMMIT}"
           echo "HEAD: ${{ github.sha }} (#${HEAD_PR})"
 
           cd src/ci/citool