pinwheel: Format nix files on save in emacs

hosts/pinwheel/modules/emacs/config.org

@@ -524,7 +524,10 @@ Setup prefix for keybindings.
            (go-mode . alex/format-on-save)
 
            (c-mode . eglot-ensure)
+
            (nix-mode . eglot-ensure)
+           (nix-mode . alex/format-on-save)
+
            (javascript-mode . eglot-ensure)
            (js-mode . eglot-ensure)
            (js-jsx-mode . eglot-ensure)