Fix Nix and test it in CI (copy #2964)

[mergify]

Our dependencies for the Nix flake were not correctly declared. This could happen because our CI job for Nix actually only tries to build clash-ghc, but the problems are in packages that clash-ghc does not depend on. So change CI to actually try to build all our packages, and run any test suites they have. (Just Cabal test-suites, not the executable clash-testsuite).

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 Fix Nix and test it in CI #2964 done by Mergify.

[mergify]

Cherry-pick of 10cb1da has failed:

On branch mergify/copy/1.8/pr-2964
Your branch is up to date with 'origin/1.8'.

You are currently cherry-picking commit 10cb1da6.
  (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)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   nix/overlay-ghc910.nix

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally