Pin Nix GHC minor version (copy #2984)

[mergify]

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

---

This is an automatic copy of pull request #2984 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 762f7f0 has failed:

On branch mergify/copy/1.8/pr-2984
Your branch is up to date with 'origin/1.8'.

You are currently cherry-picking commit 762f7f0f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   README.md
	modified:   nix/overlay.nix

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   flake.nix

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