Add sbt caching to GitHub Actions

Author: sim642

.github/workflows/sbt.yml

@@ -16,6 +16,7 @@ jobs:
       with:
         distribution: 'zulu'
         java-version: '21'
+        cache: 'sbt'
     - uses: sbt/setup-sbt@v1
     - name: Test with SBT
       run: sbt test