builds

If CPM_SOURCE_CACHE is set to an empty string, CPM fails with file DOWNLOAD error: cannot create directory '/cpm' - Specify file by full.

Saving a build with a legendary weapon causes the entry in "My builds" to look like this:

See #183

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 canno