GH Actions: use sbt/setup-sbt@v1 to install sbt

As of December 2024, `ubuntu-latest` now stands for `ubuntu-24.04`,
which unlike the previous version `ubuntu-22.04` no longer has `sbt`
preinstalled, see
https://github.com/sbt/setup-sbt/tree/v1.1.5?tab=readme-ov-file#why-is-this-github-action-needed

See also
* actions/runner-images#10767
* actions/runner-images#10788

.github/workflows/test.yml

@@ -29,5 +29,6 @@ jobs:
         with:
           distribution: 'temurin'
           java-version: ${{ matrix.java }}
+      - uses: sbt/setup-sbt@v1
       - name: Run tests
         run: sbt test