Update caching in the dependency check action

The cache strategy of setup-java does not work as I intended: It creates a cache entry once, reloads it in the subsequent runs, but once these runs finish, they don't overwrite the cache.
What does it mean for the dependency check? (n.b. my pom.xml didn't change, which makes the cache key)
- 1st run was the build-and-deploy-to-ossrh action - it created the cache.
- All subsequent runs of the dependency-check action: Take the cache from first run. Oh! There is no database, let's download and build it.

The new strategy:
Every action run creates a new cache entry with the current timestamp as key. restore-keys hopefully picks the latest cache key in the next run.

.github/workflows/maven-dependency-check.yml

@@ -7,12 +7,22 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+    - name: Get Date
+      id: get-date
+      run: |
+        echo "::set-output name=date::$(/bin/date +%s)"
+      shell: bash
+    - name: Cache local Maven repository
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2/repository
+        key: ${{ runner.os }}-maven-dependencies-check-${{ steps.get-date.outputs.date }}
+        restore-keys: ${{ runner.os }}-maven-dependencies-check-
     - name: Set up JDK 8
       uses: actions/setup-java@v2
       with:
         java-version: '8'
         distribution: 'adopt'
-        cache: 'maven'
     - id: dependency-check
       name: Check maven dependencies
       run: |