Build requires Android Build Tools <= 30.0.3

'dx' is missing in 31.0.0.

shell.nix

@@ -6,6 +6,7 @@
 let
 
   android = pkgs.androidenv.composeAndroidPackages {
+    buildToolsVersions = [ "30.0.3" ];
     platformVersions = [ "29" ];
     abiVersions = [ "armeabi-v7a" ];
   };