fix mac coverage collection

[bitfaster]

Appears that the CoverletOutput parameter is ignored on MacOS, after fixing the path can see that other params (e.g. CoverletOutputFormat are also ignored).

Results in this output:

Calculating coverage result...
  Generating report '/Users/runner/work/BitFaster.Caching/BitFaster.Caching/BitFaster.Caching.UnitTests/coverage.json'

But on Windows with the same CoverletOutput parameter, logs reflect this:

Calculating coverage result...
  Generating report 'TestResults\coverage.info'

Workarounds:

1. Hard code to expected path observed in runner logs
2. Add explicit properties to the csproj to control CollectCoverage and CoverletOutputFormat

[coveralls]

Coverage remained the same at 96.611% when pulling 43420b7 on users/alexpeck/maccover into f8627c9 on main.