chore[ci]: separate codecov upload into separate job

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

this commit separates `coverage combine` (the local tool) and codecov
upload (upload to 3rd party app) into separate steps. it also uploads
the coverage report artifacts. this makes the actions more transparent
since we can download the coverage database to inspect it if there is
an issue, and allows us to set the `coverage report` as a required CI
check, which does not depend on a 3rd party service. (if it depended on
a 3rd party service, we might then be unable to merge PRs if there is
an issue on the 3rd party service's side).

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.09%. Comparing base (9a2cb2e) to head (d853ec0).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4455   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files         119      119           
  Lines       16931    16931           
  Branches     2865     2865           
=======================================
  Hits        15593    15593           
  Misses        919      919           
  Partials      419      419           

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[cyberthirst]

approve without review