ci: fix unexpected input 'file' warning in coverage job

[cakebaker]

This PR fixes an Unexpected input(s) 'file' warning (see, for example, https://github.com/uutils/coreutils/actions/runs/15113187214/job/42477260722#step:10:1) from the codecov-action by changing the deprecated key file to files.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/cp/no-ctx is no longer failing!