Extract Codecov upload steps in CI workflow to dedicated dependent job

[lbianchi-lbl]

Summary/Motivation:

• Codecov upload failures are frustrating, as they currently require re-running the entire test suite that generates the report (20-40+ minutes)
• The proper fix is to use a Codecov upload token, saved as a repository secret
  • However, this require a major restructuring of the CI workflow because pull_request triggers cannot access secrets, requiring the creation of a new, non-pull_request workflow
  • Synchronizing triggers and passing data between different workflows is considerably more complicated compared to doing the same between different jobs in the same workflow
• As an intermediate fix, we can separate the Codecov upload so that it runs in a separate job within the pull_request-triggered workflow, so that when it fails, it can be re-run separately (which should only take a few seconds)

Changes proposed in this PR:

• In pytest job: replace step uploading report to Codecov with actions/upload-artifact
• Create new upload-coverage job dependent on pytest that just

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7f78841) 77.43% compared to head (75a4f3e) 77.43%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1334      +/-   ##
==========================================
- Coverage   77.43%   77.43%   -0.01%     
==========================================
  Files         390      390              
  Lines       63758    63758              
  Branches    11737    11737              
==========================================
- Hits        49373    49370       -3     
- Misses      11841    11845       +4     
+ Partials     2544     2543       -1     

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[lbianchi-lbl]

@ksbeattie @andrewlee94 FYI - I'm merging this in now to restore Codecov upload functionality for main and other PRs.