CI: skip code coverage upload on scheduled tests (matplotlib#27688)

* CI: install pytest pre-release on one py3.12 run

This is to keep coverage in the test suite up

* CI: solve actual problem and skip upload on pre-release jobs

Author: tacaswell

.github/workflows/tests.yml

@@ -336,6 +336,7 @@ jobs:
               -instr-profile default.profdata > info.lcov
           fi
       - name: Upload code coverage
+        if: ${{ github.event_name != 'schedule' }}
         uses: codecov/codecov-action@v3
         with:
           name: "${{ matrix.python-version }} ${{ matrix.os }} ${{ matrix.name-suffix }}"