fix: expand paths before checking for existence

[josegonzalez]

Without this, all characters in the coverage_file value are treated as literal characters in the path. The combinations of ~, ./, or ../ won't be treated as expected, causing weird build failures for users.

Note that if the file doesn't exist, then the path will resolve to empty string, which is fine.

Aside, I used a separate variable so that the logging would reflect the originally passed in value and not the resolved path.

Closes #33

[mrexox]

Thank you for the fix. You can check it in @2.2.0