Write codecov results for the head commit, not merge head

Author: alcaeus

.github/workflows/tests.yml

@@ -95,6 +95,7 @@ jobs:
           files: coverage.xml
           flags: "${{ matrix.mongodb-version }}-${{ matrix.topology }}"
           token: ${{ secrets.CODECOV_TOKEN }}
+          override_commit: "{{ github.event.pull_request.head.sha }}"
 
       - name: Upload test results to Codecov
         uses: codecov/test-results-action@v1
@@ -103,3 +104,4 @@ jobs:
           files: test-results.xml
           flags: "${{ matrix.mongodb-version }}-${{ matrix.topology }}"
           token: ${{ secrets.CODECOV_TOKEN }}
+          override_commit: "{{ github.event.pull_request.head.sha }}"