prepend the `v`

Update version of `checkout` action.

Correct some incorrect references to `lcov`, which seem to imply it as the only supported coverage report format, leftover from `v1` docs.

Replaces #198 and closes #197

* Add `build-number` to input options to let users override the action's default build number: `${{ github.run_id }}`.
 
Co-authored-by: Brian Torres <briantorres@getgather.com>

* Add test for coverage-reporter-version.
* Add logic to handle when coverage-reporter-version is empty, in addition to when it is null/unset.
* Apply fail-on-error logic to error when user tries to set coverage-reporter-version on MacOS (not available there).