Upload to codecov only once all_tests are run

[ricardoV94]

Description

Following #6654 we can now wait until all tests have run before uploading the coverage report to codecov.

This will avoid the premature bot message saying coverage has dropped that happens all the time as the initial partial coverage files are uploaded by each individual job.

PyTensor can probably be used as a template: https://github.com/pymc-devs/pytensor/blob/98de246293855def134b86b5af6599e74a44835b/.github/workflows/test.yml#L235-L262

[michaelraczycki]

I'd like to work on that

[ricardoV94]

Hopefully this will also fix this strange behavior noted by @bwengals #6801 (review)