ci(android): set $ENV{ANDROID_NDK} for CMake

Author: rami3l

ci/docker/android/Dockerfile

@@ -5,6 +5,7 @@ RUN apt-get update && apt-get install -qy gcc-multilib libclang-dev \
   && mv $HOME/.cargo/bin/bindgen /usr/bin
 
 ENV PATH=$PATH:/android/ndk/toolchains/llvm/prebuilt/linux-x86_64/bin \
+    ANDROID_NDK=/android/ndk/ \
     AR_arm_linux_androideabi=llvm-ar \
     AR_armv7_linux_androideabi=llvm-ar \
     AR_aarch64_linux_android=llvm-ar \