Checkout correct git commit for pull requests on Github Actions CI

By default git commit for pull requests on Github Actions CI is the fake
merge commit, not the pushed one. So checkout the correct pushed (HEAD)
commit.

.github/workflows/ci.yaml

@@ -256,10 +256,16 @@ jobs:
       fail-fast: false
       matrix: ${{ fromJson(needs.build-matrix.outputs.matrix) }}
     steps:
-      - name: Checkout git commit ${{ github.sha }}
+      - name: Checkout git commit ${{ github.sha }} (not pull request)
+        if: github.event_name != 'pull_request'
         uses: actions/checkout@v3
         with:
           ref: ${{ github.sha }}
+      - name: Checkout git commit ${{ github.event.pull_request.head.sha }} (pull request)
+        if: github.event_name == 'pull_request'
+        uses: actions/checkout@v3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
       - name: Setup cpan sources cache
         uses: actions/cache@v3
         with: