fix ci-base: install Node 20 via binary tarball instead of nodesource apt

nodesource setup_20.x on Ubuntu 24.04 installs Ubuntu's nodejs 18 (no npm)
instead of Node 20. Binary tarball is reliable and includes npm.

servers/legion/k8s/gitea-runner/Dockerfile

@@ -29,11 +29,13 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     zstd \
     && rm -rf /var/lib/apt/lists/*
 
-# Node.js 20 LTS — required to execute Forgejo JS actions (checkout, upload-artifact, cache, etc.)
-RUN curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
-    && apt-get install -y nodejs \
-    && npm install -g yarn \
-    && rm -rf /var/lib/apt/lists/*
+# Node.js 20 LTS via binary tarball (nodesource apt repo is unreliable on Ubuntu 24.04)
+RUN NODE_VERSION=20.19.1 \
+    && curl -fsSL "https://nodejs.org/dist/v${NODE_VERSION}/node-v${NODE_VERSION}-linux-x64.tar.xz" \
+       | tar -xJ -C /usr/local --strip-components=1 \
+    && node --version \
+    && npm --version \
+    && npm install -g yarn
 
 # Python 3 + pip + venv
 RUN apt-get update && apt-get install -y --no-install-recommends \