fix ndk build issue (#366)

* u3d version?

* more print out for u3d

* fix ndk build issue

Co-authored-by: Cynthia Jiang <cynthiajiang@google.com>

Author: cynthiajoan

.github/workflows/build_android.yml

@@ -74,6 +74,24 @@ jobs:
         with:
           python-version: '3.7'
 
+      - name: Cache NDK
+        id: cache_ndk
+        uses: actions/cache@v2
+        with:
+          path: /tmp/android-ndk-r21e
+          key: android-ndk-${{ matrix.os }}-r21e
+
+      - name: Check cached NDK
+        shell: bash
+        if: steps.cache_ndk.outputs.cache-hit != 'true'
+        run: |
+          # If the NDK failed to download from the cache, but there is a
+          # /tmp/android-ndk-r21e directory, it's incomplete, so remove it.
+          if [[ -d "/tmp/android-ndk-r21e" ]]; then
+            echo "Removing incomplete download of NDK"
+            rm -rf /tmp/android-ndk-r21e
+          fi
+
       - name: Install prerequisites
         shell: bash
         run: |
@@ -82,13 +100,8 @@ jobs:
           python scripts/gha/install_prereqs_desktop.py
           build_scripts/android/install_prereqs.sh
           cd ..
-
-      - name: Cache NDK
-        id: cache_ndk
-        uses: actions/cache@v2
-        with:
-          path: /tmp/android-ndk-r21e
-          key: android-ndk-${{ matrix.os }}-r21e
+          echo "NDK_ROOT=/tmp/android-ndk-r21e" >> $GITHUB_ENV
+          echo "ANDROID_NDK_HOME=/tmp/android-ndk-r21e" >> $GITHUB_ENV
 
       - name: Install Unity installer (U3D)
         shell: bash