flake: format the tree with one treefmt

The pre-commit hooks kept their own list of formatters, so nixpkgs-fmt in
a hook and nixfmt in the check disagreed the moment flake-checks moved.
`nix fmt` is now built from the same treefmt module the formatting check
is, and the hook and the Makefile both call it.

.prettierignore

@@ -1,2 +1,5 @@
 .github/workflows/test-integration-v2*
 docs/
+
+# Golden fixtures are byte-compared; a formatter must not rewrite them.
+hscontrol/testdata/