Unset ANDROID_HOME and ANDROID_NDK_HOME on the macos platform (#2882)

Our Intel Mac machines (`macos` platform) have an outdated Android SDK
that doesn't work with the latest rules_android, so unset `ANDROID_HOME`
and `ANDROID_NDK_HOME` for all builds on that platform.

The variables are unset before the task's `environment` is applied, so
pipelines can still set them explicitly if needed. `macos_arm64` is
unaffected.