Write a nix expression to enter a shell with specified build artifact

It should be quite straightforward to write a nix expression which is parameterised by a CI job which downloads the artifact and enters into a nix shell which provisions that version of GHC.

To be precise, consider the recent job: https://gitlab.haskell.org/ghc/ghc/-/jobs/6427.

It built these artifacts: https://gitlab.haskell.org/ghc/ghc/-/jobs/6427/artifacts/browse

We should download the bindist and patch it suitably so it works like a normal GHC release.

Some inspiration can probably be drawn from the scripts that Ben wrote for the head.hackage repo: https://github.com/hvr/head.hackage/blob/master/default.nix

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Trac field	Value
Version	8.6.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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