Install Node in the backend CI job before checkout

The frontend job succeeded after the runner HOME fix, but backend failed at
a different point: actions/checkout is a JavaScript action and needs a Node
runtime inside the job container to run it at all (exec: "node": executable
file not found in $PATH), and hseeberger/scala-sbt has no Node installed.
Same lesson HOP's own CI setup already documented ("JavaScript actions need
Node in the job image") -- mirroring their fix: install git+nodejs via apt
before the checkout step.

.gitea/workflows/ci.yml

@@ -15,6 +15,11 @@ jobs:
     container:
       image: hseeberger/scala-sbt:17.0.2_1.6.2_2.13.8
     steps:
+      - name: Install checkout runtime
+        run: |
+          apt-get update
+          apt-get install --yes --no-install-recommends git nodejs
+          rm -rf /var/lib/apt/lists/*
       - uses: actions/checkout@v4
       - name: sbt test
         run: sbt -batch test