Add setup-sbt to conformance

Author: thesamet

.github/workflows/test.yml

@@ -54,6 +54,7 @@ jobs:
       with:
           key: conformance-${{ runner.os }}-${{hashFiles('conformance/build_test_runner.sh')}}
           path: /tmp/conformance-test-runner
+    - uses: sbt/setup-sbt@v1
 
     # Build conformance_test_runner on a cache miss
     - uses: bazelbuild/setup-bazelisk@v3