ci/check-size: checkout HEAD commit instead of merge commit

By default, the SHA that is checked out is a merge commit from the base
branch on the PR. To use the PR HEAD instead of this merge commit, we
need to explicitly specify the SHA from the event context provided by
GitHub.

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

.github/workflows/check-size.yml

@@ -14,6 +14,8 @@ jobs:
       with:
         # Needed to rebase against the base branch
         fetch-depth: 0
+        # Checkout pull request HEAD commit instead of merge commit
+        ref: ${{ github.event.pull_request.head.sha }}
     - name: Configure git user details
       run: |
         git config --global user.email "checkpoint-restore@users.noreply.github.com"