Install a real Node 20 in the backend CI job, not Debian bullseye's default

Node got installed (previous fix worked) but actions/checkout@v4's bundled
JS uses modern syntax (class static blocks) requiring Node >=16.11.
hseeberger/scala-sbt is Debian bullseye-based, whose default apt "nodejs"
package is a stale Node 12 -- SyntaxError: Unexpected token '{' at "static
{". HOP's own image is bookworm-based (newer default), which is why they
never hit this. Use NodeSource's setup script to get an actual current Node
20 regardless of the distro's packaged version.

.gitea/workflows/ci.yml

@@ -18,7 +18,9 @@ jobs:
       - name: Install checkout runtime
         run: |
           apt-get update
-          apt-get install --yes --no-install-recommends git nodejs
+          apt-get install --yes --no-install-recommends curl ca-certificates git
+          curl -fsSL https://deb.nodesource.com/setup_20.x | bash -
+          apt-get install --yes --no-install-recommends nodejs
           rm -rf /var/lib/apt/lists/*
       - uses: actions/checkout@v4
       - name: sbt test