Add support for HEAD builds

The gitlab CI infrastructure builds every commit on a wide number of platforms. It would be useful if users could test out various HEAD builds in some fashion using ghcup.

For example:

https://gitlab.haskell.org/ghc/ghc/-/jobs/artifacts/master/download?job=validate-x86_64-linux-deb8

Will get fetch the most recent successful debian 8 build.

There are similar links for OS X, windows and other platforms.

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