Update agru version label to match the locked v0.2.1 revision

flake.lock already pins agru at the v0.2.1 commit (via #5415);
this label only names the built derivation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

flake.nix

@@ -22,7 +22,7 @@
         };
         agru = pkgs.buildGo125Module {
           pname = "agru";
-          version = "0.1.19";
+          version = "0.2.1";
           src = agru-src;
           vendorHash = null;
         };