Flaky nix shell check in CI

[goodlyrottenapple]

The CI Nix shell check action often errors out with

error: derivation '/nix/store/58xgc5mhip4f2bfc5fpb7smm7b4fzl9y-hoogle-local-0.1.drv' requires non-existent output 'doc' from input derivation '/nix/store/027ci16pwznr0krbh62ik4rhm4l8lf7j-pipes-aeson-lib-pipes-aeson-0.4.1.8-haddock.drv'

This is most likely due to NixOS/nix#6572
The options to fix are:

• separate this step in the run into its own action and run via an older version of nix
• downgrade nix for the whole action (tho this may not be feasible if this behaviour occurs in nix >2.4)
• or drop the shell check entirely

[jberthold]

Suggestion:

• replacenix-shell check by nix develop check
• make nix develop support fourmolu and integration tests
• remove old-style nix-* commands completely and update the README.md, except for nix-build test.nix

[ana-pantilie]

@goodlyrottenapple found that the third bullet point (remove old-style nix-* commands completely and update the README.md, except for nix-build test.nix) is not possible to do currently.

#3245 fixes the rest.