How to inform users there's a newer metadata?

[hasufell]

The metadata format is subject to change, e.g. when we add a new distro constructor or add new fields etc.

At the moment, GHCup relies on the fact that the URL is compiled into the program:

ghcup-hs/lib/GHCup/Version.hs

Line 37 in d143dae

ghcupURL = [uri|https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-0.0.7.yaml|]

However, if users use the ghcup add-release-channel feature (e.g. for nightlies), they might not know they're running an outdated metadata variant.

I can see a couple of ways to resolve that:

1. Manual: check for newer file versions via some brute-force mechanism in the proposed healthcheck command
2. Automatic: add deprecation info into the metadata file itself, possibly with a link to the new URL. On startup, GHCup can warn about it.

[hasufell]

@mpickering since relevant for nightlies

[hasufell]

Done