Drop into a nix shell with ghcHEAD

[Matthew Pickering]

Hi all,

I have written a devastating new script which makes it easy to test
snippets and packages with build artefacts built by the gitlab CI. One
invocation of `nix-shell` will enter an environment with the most
recent build artefact from master.

Usage:

```
> nix-shell https://github.com/mpickering/ghc-artefact-nix/archive/master.tar.gz
unpacking 'https://github.com/mpickering/ghc-artefact-nix/archive/master.tar.gz'...
<lots more output>
> ghc --version
The Glorious Glasgow Haskell Compilation System, version 8.7.20190115
```

Repo: https://github.com/mpickering/ghc-artefact-nix

So far we have tested it works on NixOS and darwin but support for
other platforms that nix supports and we build artefacts for should be
easy. It might already work on i386 and aarch64 as well but I didn't
test these.

Cheers,

Matt