Cache ~/.m2 dir for GitHub builds.

Author: Gary Gregory

.github/workflows/maven.yml

@@ -38,6 +38,12 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2.3.2
+    - uses: actions/cache@v2
+      with:
+        path: ~/.m2/repository
+        key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
+        restore-keys: |
+          ${{ runner.os }}-maven-
     - name: Set up JDK ${{ matrix.java }}
       uses: actions/setup-java@v1.4.0
       with: