Change file to files codecov action

[mcbarton]

Description

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

We are currently getting the following warning message in the main 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
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (ca05e91) to head (8ac2e29).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #331   +/-   ##
=======================================
  Coverage   81.78%   81.78%           
=======================================
  Files          20       20           
  Lines         950      950           
  Branches       87       87           
=======================================
  Hits          777      777           
  Misses        173      173           

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[mcbarton]

As can be seen here the codecov action still works, but we no longer have this warning https://github.com/compiler-research/xeus-cpp/actions/runs/15420459840