Include building the Android template in the flake

[goatchurchprime]

At the moment the flake downloads the precompiled Android template for each version as linked to from the blog as each snapshot is announced: https://godotengine.org/download/archive/4.5-dev4/

Sometimes I need to use a PR version of Godot, for example to get the microphone working on Android: godotengine/godot#105244

It would be really useful if we could build the Android template from source in this flake as well. Unfortunately when I did it I hit this snag with the exact versions of the NDK: tadfisher/android-nixpkgs#113 I don't know enough nixos to fix this.

At present my only option has been to compile the Android templates on a spare Windows machine and then copy them across to the linux machine.