Fix CI test workflow using pinned Android NDK

.github/workflows/test.yaml

@@ -4,11 +4,20 @@ on: [push, pull_request]
 
 env:
   ANDROID_NDK_ROOT: /usr/local/lib/android/sdk/ndk/21.4.7075529
+  # By default the new ubuntu-20.04 images use the following ANDROID_NDK_ROOT
+  # ANDROID_NDK_ROOT: /usr/local/lib/android/sdk/ndk/25.0.8775105
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
+      - name: Install Android NDK 21.4.7075529
+        run: |
+          ANDROID_ROOT=/usr/local/lib/android
+          ANDROID_SDK_ROOT=${ANDROID_ROOT}/sdk
+          SDKMANAGER=${ANDROID_SDK_ROOT}/cmdline-tools/latest/bin/sdkmanager
+          echo "y" | $SDKMANAGER "ndk;21.4.7075529"
+
       - name: Check out PR branch
         uses: actions/checkout@v2