Update to the latest setup-scala.

This release automatically sets the environment variable `CI=true` so
that CI-related logic gets picked up the same way it does on Travis.
I'm surprised this variable is not set by default in GitHub Actions.

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: windows-latest
     steps:
       - uses: actions/checkout@v1
-      - uses: olafurpg/setup-scala@v5
+      - uses: olafurpg/setup-scala@v6
       - name: Run unit tests
         run: bin/test.sh "unit/test"
         shell: bash