Update breaking change check to checkout the SHA instead of the ref (#4598)

Author: JamesBurnside

.github/workflows/breaking-change-check.yml

@@ -69,7 +69,7 @@ jobs:
       # Checkout the branch to be merged into in PR
       - name: Checkout base branch
         if: ${{ github.event_name == 'pull_request' && !contains(github.event.pull_request.labels.*.name, 'breaking-change') }}
-        run: git checkout ${{ github.event.pull_request.base.ref }}
+        run: git checkout ${{ github.event.pull_request.base.sha }}
       # Install dependencies for the base branch
       - name: Install beta dependencies
         if: ${{ matrix.flavor == 'beta' && github.event_name == 'pull_request' && !contains(github.event.pull_request.labels.*.name, 'breaking-change') }}