🐛 Fix sdl2/sdl3 bootstrap conflict in CI

[AndreMiras]

Auto-detect sdl3 bootstrap when any SDL3-related recipe (sdl3, sdl3_image, sdl3_mixer, sdl3_ttf) is in requirements, preventing conflict with hardcoded sdl2 bootstrap in the test app's setup.py.

Also remove stale commented-out URL from sdl3_mixer recipe that incorrectly referenced SDL_ttf.

[AndreMiras]

It seems to be working, see example build log for this change:

[INFO]:    Building updated recipes for the following archs: ['arm64-v8a']
[INFO]:    recipes modified: {'sdl3_mixer'}
[INFO]:    recipes to build: {'sdl3_mixer'}
[INFO]:    recipes to build (no broken): {'sdl3_mixer'}
[INFO]:    requirements: python3,sdl3_mixer
[INFO]:    Detected sdl3 recipe in requirements, using sdl3 bootstrap
[INFO]:    Build command: setup.py apk --sdk-dir /Users/runner/.android/android-sdk --ndk-dir /Users/runner/.android/android-ndk --requirements python3,sdl3_mixer --bootstrap sdl3 --arch=arm64-v8a

More particularly these lines:

[INFO]:    Detected sdl3 recipe in requirements, using sdl3 bootstrap
...
[INFO]:    Build command: setup.py apk --sdk-dir /Users/runner/.android/android-sdk --ndk-dir /Users/runner/.android/android-ndk --requirements python3,sdl3_mixer --bootstrap sdl3 --arch=arm64-v8a