Update codecov action to use files instead of file

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

We are currently getting the following warning message in the coverage workflow. It seems the file key in codecov has changes to files in a newer version. This PR fixes that, and should no longer have this warning.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[mcbarton]

This change is also being made in xeus-cpp here compiler-research/xeus-cpp#331

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.58%. Comparing base (7f33134) to head (9ecebb1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #613   +/-   ##
=======================================
  Coverage   77.58%   77.58%           
=======================================
  Files           9        9           
  Lines        3734     3734           
=======================================
  Hits         2897     2897           
  Misses        837      837           

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[vgvassilev]

Lgtm!