ci: use coverage from PyPI on Ubuntu
I'm seeing warnings like this: CoverageWarning: Couldn't use data file '/builds/mercurial/hg-git/coverage-279600': Looks like a coverage 4.x data file. Are you mixing versions of coverage?
This instance will be upgraded to Heptapod 0.31.0 (final) on 2022-05-24 at 14:00 UTC+2 (a few minutes of down time)
I'm seeing warnings like this: CoverageWarning: Couldn't use data file '/builds/mercurial/hg-git/coverage-279600': Looks like a coverage 4.x data file. Are you mixing versions of coverage?