add treefmt check

nix/checks/treefmt.nix

@@ -0,0 +1,22 @@
+{
+  runCommandNoCC,
+  gofumpt,
+  alejandra,
+  python3,
+  treefmt,
+}:
+runCommandNoCC "treefmt" {
+  nativeBuildInputs = [
+    gofumpt
+    treefmt
+    alejandra
+    python3.pkgs.flake8
+    python3.pkgs.black
+  ];
+} ''
+  # keep timestamps so that treefmt is able to detect mtime changes
+  cp --no-preserve=mode --preserve=timestamps -r ${../..} source
+  cd source
+  HOME=$TMPDIR treefmt --no-cache --fail-on-change
+  touch $out
+''