Add Maven caching to PR Builder [DI-336] (#677)

Because it's `pull_request_target`, the cache _should_ be accessible to
improve build times between branches.

To be tested post-merge.

Author: JackPGreen

.github/workflows/builder.yaml

@@ -33,6 +33,7 @@ jobs:
         with:
           java-version: 17
           distribution: temurin
+          cache: maven
       - name: Build and test
         run: |
           ${RUNNER_DEBUG:+set -x}