Implement actions/cache for maven packages

[TheBusyBiscuit]

To speed up build times, we should implement the actions/cache GitHub action for maven packages here.
An example would look like this:

    - name: Cache Maven packages
      uses: actions/cache@v2
      with:
        path: ~/.m2
        key: ${{ runner.os }}-m2-${{ hashFiles('**/pom.xml') }}
        restore-keys: ${{ runner.os }}-m2

The key problem here is that we cannot use **/pom.xml as this is a JavaScript project first and foremost, we could use another file as a key here.

Workflow file:
https://github.com/TheBusyBiscuit/builds/blob/gh-pages/.github/workflows/build.yml