Reduce Codecov failure rate by using a token

Try to use Codecov token to avoid the recurring Codecov 404 errors.
Codecov suggests the use of tokens even for public repositories when
such failures occur:
https://community.codecov.com/t/upload-issues-unable-to-locate-build-via-github-actions-api/3954.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -885,6 +885,7 @@ jobs:
       - name: Upload coverage statistics to Codecov
         uses: codecov/codecov-action@v3
         with:
+          token: ${{ secrets.CODECOV_TOKEN }}
           files: build/html/coverage.info
           fail_ci_if_error: true
           verbose: true