Allow to statically overwrite distro detection

People on e.g. PopOS, which is a strict ubuntu derivate, have trouble using ghcup, because it picks suboptimal bindists.

We should provide a way to allow users to select a compatible distro manually to overwrite the distro detection.

There's already a hidden switch ghcup install ghc --platform x86_64-fedora27-linux latest, but this should be static configuration.