Cache the gradle dependency cache

This'll preserve the FG and general module cache. Note, this doesn't
include gradle wrappers, but that's normally pretty fast to download.

.github/workflows/main-ci.yml

@@ -15,6 +15,14 @@ jobs:
       with:
         java-version: 1.8
 
+    - name: Cache gradle dependencies
+      uses: actions/cache@v1
+      with:
+        path: ~/.gradle/caches
+        key: ${{ runner.os }}-gradle-${{ hashFiles('gradle.properties') }}
+        restore-keys: |
+          ${{ runner.os }}-gradle-
+
     - name: Build with Gradle
       run: ./gradlew build --no-daemon