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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