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.
Introduces treefmt with a simple nixfmt-rfc-style configuration to
format all Nix files.
This is only practically usable with the following commit that formats
all files accordingly.
Also post a comment in case base branch is wrong
This guides newcomers in how to smoothly handle the potentially scary
situation of having thousands of commits listed in a PR.
While CI shows the same, people might not even look at CI if the PR
looks botched.
