Use separate CI job for Codecov upload

[lbianchi-lbl]

Summary/Motivation

Resolve part of #1364

Changes proposed in this PR:

• Move Codecov upload step to own separate job in CI workflow

Legal Acknowledgement

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1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.83%. Comparing base (c93438e) to head (15a030c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1365      +/-   ##
==========================================
- Coverage   93.83%   93.83%   -0.01%     
==========================================
  Files         337      338       +1     
  Lines       35290    35354      +64     
==========================================
+ Hits        33114    33174      +60     
- Misses       2176     2180       +4     

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