Add continue-on-error for cache step, and upgrade GH cache action (#3133)

Author: rnorth

.github/workflows/ci.yml

@@ -46,7 +46,8 @@ jobs:
         with:
           java-version: '1.8'
       - name: Cache Gradle Home files
-        uses: actions/cache@v1
+        uses: actions/cache@v2
+        continue-on-error: true
         with:
           path: ~/.gradle/caches
           key: ${{ runner.os }}-gradle-home-${{matrix.gradle_args}}_check-${{ hashFiles('**/*.gradle') }}