Codecov upload failures

[lbianchi-lbl]

Problem

• Codecov upload have been failing in a way that's hard to detect (see e.g. https://github.com/watertap-org/watertap/pull/1363#issuecomment-2070915795)

Solution

• [x] Short/medium-term: move the Codecov upload step to a separate dependent job so when it fails it can be restarted without having to re-run the test suite
• [ ] Long-term: move the Codecov upload step to a separate dependent workflow that is able to access repository secrets so that a dedicated Codecov auth token can be used (as opposed to relying on the GHA "no tokens needed" default we have now, which is likely what's being causing the failures)

[lbianchi-lbl]

Update: this is an example of the type of error that would require the 2nd checkbox above to resolve: