ci: explicitly install sbt

as it doesn't come with ubuntu-latest anymore

Author: raboof

.github/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
           distribution: temurin
           java-version: ${{matrix.java}}
       - uses: coursier/cache-action@v6
+      - uses: sbt/setup-sbt@v1
       # note use of an old sbt version, as a smoke test
       - run: "sbt +test mimaReportBinaryIssues 'set sbtplugin/scriptedSbt := \"1.3.13\"' 'scripted sbt-mima-plugin/minimal'"
   testFunctional: