[BUG] Git files (network) not found when run from a non-git folder.

[spalmurray]

Describe the bug

I tried uploading a coverage file from GitHub Actions and noticed that the path fixup didn't work anymore.
I traced that to this issue: GitVersioningSystem.is_available only returns True if the current directory is a git repository

So even if --network-root-folder was specified and set to a git repository it will not be detected because GitVersioningSystem is not used as it fails if the current folder is not a git repo and hence git rev-parse --show-toplevel fails to early.

To Reproduce
Steps to reproduce the behavior:

1. Go to an empty folder
2. run the app with --network-root-folder set correctly
3. See that "relevant files" is empty and fixups are not done

codecov/codecov-cli#624