This enables `nix fmt`, though it won't be practically usable without
also reformatting all files, which is done in a following commit.

(cherry picked from commit 5a8296d)

Changes the Nix format checking workflow to now strictly enforce
formatting of all Nix files using the treefmt setup introduced
in the pre-previous commit.

This is in [accordance with the approved RFC 166](https://github.com/NixOS/rfcs/blob/master/rfcs/0166-nix-formatting.md#reformat-nixpkgs).

Note that the "skip treewide" thing is no longer necessary, already
before, because there's nothing that would fail for treewide changes.
Previously the problem was that the GitHub API would be bombarded.

(cherry picked from commit 927521a)

(cherry picked from commit 2140bf3)

Format all Nix files using the officially approved formatter,
making the CI check introduced in the previous commit succeed:

  nix-build ci -A fmt.check

This is the next step of the of the [implementation](NixOS/nixfmt#153)
of the accepted [RFC 166](NixOS/rfcs#166).

This commit will lead to merge conflicts for a number of PRs,
up to an estimated ~1100 (~33%) among the PRs with activity in the past 2
months, but that should be lower than what it would be without the previous
[partial treewide format](NixOS#322537).

Merge conflicts caused by this commit can now automatically be resolved while rebasing using the
[auto-rebase script](https://github.com/NixOS/nixpkgs/tree/8616af08d915377bd930395f3b700a0e93d08728/maintainers/scripts/auto-rebase).

If you run into any problems regarding any of this, please reach out to the
[formatting team](https://nixos.org/community/teams/formatting/) by
pinging @NixOS/nix-formatting.

(branch-equivalent to commit 374e6bc)

And enable use of the auto-rebase script: https://github.com/NixOS/nixpkgs/tree/master/maintainers/scripts/auto-rebase

(branch-equivalent to commit 49cf547)