CI: use a token for the Codecov action to upload (#657)

This is required now, it was a breaking change in
`codecov-action@v4`. Follows up on a comment on
PR 479.

Author: rgommers

.github/workflows/tests.yml

@@ -138,6 +138,8 @@ jobs:
       - name: Upload coverage report
         uses: codecov/codecov-action@v4
         if: ${{ always() }}
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
 
   msvc:
     runs-on: windows-latest