Support uninstall & reinstall `force` option

[sdankel]

It would be great to have an option to uninstall any existing version of nix before installing the new version. Something like a force option.

Uninstallation is complicated, particularly on macos: NixOS/nix#3900

Having a github action to do this would be very helpful.

[domenkozar]

Is this for self-hosted runners?

[sdankel]

Yes, it would be useful for self-hosted runners.

[pchandra19]

I’d really appreciate this option, as it would help users of self-hosted runners. My workflow failed once due to a minor mistake, and now installing Nix via cachix/install-nix-action@v31.3.0 always fails—even after I’ve fully cleaned Nix from the runner.