ci: only check last commit for API schema in PRs (#3133)

### Description

PRs are intended to be squashed to a single commit. Only checking the
last commit gives us the intended state of the repo and ensures that if
the author commits the schema fixes later, the CI passes as expected.
Currently, the CI will fail because the earlier commits still have an
out of date schema.

.github/workflows/api-schema.yml

@@ -42,7 +42,7 @@ jobs:
 
       - name: Get pull request commit range
         if: github.event_name == 'pull_request'
-        run: echo "COMMIT_RANGE=origin/${{ github.base_ref }}..${{ github.sha }}" >> $GITHUB_ENV
+        run: echo "COMMIT_RANGE=${{ github.sha }}~1...${{ github.sha }}" >> $GITHUB_ENV
 
       - name: Get push commit range
         if: github.event_name == 'push'