refactor(depotfmt): Move formatting check into an extra step

Change-Id: I7e4cf6bb2351bd11a5396f1663c0d4cc97c0d94e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5009
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>

tools/depotfmt.nix

@@ -35,5 +35,9 @@ let
       --tree-root .
   '';
 in depotfmt.overrideAttrs(_: {
-  passthru.check = check;
+  passthru.meta.ci.extraSteps.check = {
+    label = "depot formatting check";
+    command = check;
+    alwaysRun = true;
+  };
 })