set ndk env variable (#5465)

# Objective

- Fixes #5463
- set ANDROID_NDK_ROOT
- GitHub recently updated their ubuntu container, removing some of the android environment variable: actions/runner-images@ca5d04c
- `cargo-apk` is not reading the new environment variable: https://github.com/rust-windowing/android-ndk-rs/blob/9a8be258a951e213f4d07abe3c4c190d568c9694/ndk-build/src/ndk.rs#L33-L38
- this also means CI will now use the latest android NDK, I don't know if that's an issue

Author: mockersf

.github/workflows/validation-jobs.yml

@@ -59,7 +59,7 @@ jobs:
         run: cargo install --force cargo-apk
 
       - name: Build APK
-        run: cargo apk build --example android_example
+        run: ANDROID_NDK_ROOT=$ANDROID_NDK_LATEST_HOME cargo apk build --example android_example
 
   run-examples-on-windows-dx12:
     runs-on: windows-latest