workflows/editorconfig: drop and move to treefmt

We already have treefmt running for nixfmt, so it's easy to just add
another formatter to it. This gives a much better UX, because all
formatting errors are reported through the same channel.

It also saves us one CI job, which takes most of the time to just set up
the machine, clone the repo and download Nix - while doing a minimum of
actual work.

Total execution time for treefmt is ~10% slower:
- 38s only nixfmt
- 43s nixfmt + editorconfig-checker

ci/default.nix

@@ -48,6 +48,13 @@ let
         # the default formatter for Nix code.
         # See https://github.com/NixOS/nixfmt
         programs.nixfmt.enable = true;
+
+        settings.formatter.editorconfig-checker = {
+          command = "${pkgs.lib.getExe pkgs.editorconfig-checker}";
+          options = [ "-disable-indent-size" ];
+          includes = [ "*" ];
+          priority = 1;
+        };
       };
       fs = pkgs.lib.fileset;
       nixFilesSrc = fs.toSource {