Pin Nix GHC minor version

[jaschutte]

Pins down the GHC version down to the minor version. As discussed in a PR in clash-cores
(clash-lang/clash-cores#44 (comment)).

Each supported GHC version is now postfixed with a minor version number. However the overlays are not. The last digit of the version number gets stripped when Nix imports an overlay. This way we can have one set of GHC version numbers pin down the right GHC version and import the right overlay.

I nix build .#clash-ghc with all three versions succesfully.

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[jaschutte]

The pinned versions for ghc96 and ghc98 aren't the newest versions. This is because the nixpkgs input is almost a year old by now and it does not have the latest minor versions of GHC. I tried updating the nix flake, however then brick_70_1 is no longer available and that bricks everything. So I ended up leaving it as-is and maybe address it in a future PR (maybe).

[martijnbastiaan]

@kloonbot run_ci 762f7f0

[martijnbastiaan]

@jaschutte If you make a branch on this repo CI will automatically run without maintainer involvement

[DigitalBrains1]

@jaschutte If you make a branch on this repo CI will automatically run without maintainer involvement

Not only that, it'll also save running the expensive GitHub CI checks we only run for external contributions.

By the way, it doesn't need a maintainer to invoke kloonbot. Anyone in the set of owners, members of the clash-lang organisation and collaborators can do that. James is a member of clash-lang.