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.