Always use Git Credential Manager on Linux

[adil192]

The feature request

Since https://github.com/desktop/desktop/pull/18700 landed GCM support, we could now use it to bypass oath issues with the Linux client.

Proposed solution

On Linux, always use GCM, even with GitHub repos.

Additional context

This is the message I get whenever I try to push or pull:

[theofficialgman]

maybe overkill but I agree it would be nice for organization owned repos as it would fix this issue https://github.com/shiftkey/desktop/issues/217