refactor(go): nixify 'GOPATH' environment variable

home/modules/go/default.nix

@@ -5,18 +5,18 @@
   ...
 }:
 
-let
-  cfg = config.go;
-in
 {
   options.go = {
     enable = lib.mkEnableOption "Go";
   };
 
-  config = lib.mkIf cfg.enable {
-    home.packages = with pkgs; [
-      go
-      gopls
-    ];
+  config = lib.mkIf config.go.enable {
+    home = {
+      sessionVariables.GOPATH = "${config.xdg.dataHome}/go";
+      packages = with pkgs; [
+        go
+        gopls
+      ];
+    };
   };
 }