Setup sbt explicitly in GitHub Actions (#92)

Author: takezoe

.github/workflows/scala.yml

@@ -29,6 +29,8 @@ jobs:
       with:
         java-version: 11
         distribution: "temurin"
+    - name: Setup SBT
+      uses: sbt/setup-sbt@v1        
     - name: Run tests
       env:
         TEST_DIR: ${{ matrix.test_dir }}