Add Maven dependency caching on CI action

Author: simonoakesepimorphics

.github/workflows/build.yaml

@@ -11,6 +11,12 @@ jobs:
         uses: actions/setup-java@v1
         with:
           java-version: 1.8
+      - name: Cache Maven packages
+        uses: actions/cache@v2
+        with:
+          path: ~/.m2
+          key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
+          restore-keys: ${{ runner.os }}-m2
       - name: Install NetCDF tools
         run: sudo apt install netcdf-bin
       - name: Maven build and test