Use codecov action for coverage

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

It seems that coverage reporting is still off in that it uses the wrong commit to identify the report. This tries to fix it by using the codecov Github action.

[github-actions]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#fix-coverage-reporting

or load it into the REPL:
https://rollupjs.org/repl/?pr=4046

[codecov]

Codecov Report

Merging #4046 (2f47122) into master (ef807c9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4046   +/-   ##
=======================================
  Coverage   97.48%   97.48%           
=======================================
  Files         192      192           
  Lines        6788     6788           
  Branches     1996     1996           
=======================================
  Hits         6617     6617           
  Misses         84       84           
  Partials       87       87           

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