ci/android: rename variable ANDROID_NDK to ANDROID_NDK_VERSION

Rename the variable ANDROID_NDK to ANDROID_NDK_VERSION and only use it
to specify the version itself.

This makes it consistent with ANDROID_SDK_VERSION and it is also in
preparation for using the same variable to build ANGLE in the future.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33188>
4 files changed