it's hard to use the build output apk like that because it's not checked in

so use the apk derivation directly!  yea!

flake.nix

@@ -114,7 +114,7 @@
             platformVersion = "31";
             abiVersion = "x86_64";
             systemImageType = "default";
-            app = ./app/app/build/outputs/apk/debug/app-debug.apk;
+            app = self.packages.${system}.apk;
             package = "MyApp";
             activity = "MainActivity";
           };