Cache Maven artifacts to reduce the incidence of plugin resolution failure.

.github/workflows/test.yml

@@ -12,5 +12,12 @@ jobs:
         uses: actions/setup-java@v1
         with:
           java-version: 11
+      - name: Cache local Maven repository
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2/repository
+          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+          restore-keys: |
+            ${{ runner.os }}-maven-
       - name: Build with Maven
-        run: mvn -e -B package
+        run: mvn -e -B package