Use setup-scala to provide sbt

sbt has been removed from Github's base images. The recommendation from
DevX is to switch to using the setup-scala action to set up Java and Scala, along with sbt.

.github/workflows/ci.yaml

@@ -15,13 +15,12 @@ jobs:
         with:
           role-to-assume: ${{ secrets.GU_RIFF_RAFF_ROLE_ARN }}
           aws-region: eu-west-1
-      - uses: actions/setup-java@v3
-        with:
-          java-version: "11"
-          distribution: "corretto"
+      # See https://github.com/guardian/setup-java
+      - name: Setup Java and sbt
+        uses: guardian/setup-scala@v1
       - uses: actions/setup-node@v3
         with:
-          node-version-file: '.nvmrc'
+          node-version-file: ".nvmrc"
       - name: Build Pluto lambda
         run: |
           ./scripts/pluto-ci.sh
@@ -31,5 +30,3 @@ jobs:
       - name: Compile Scala and upload artifacts to RiffRaff
         run: |
           ./scripts/scala-ci.sh
-
-