fix(ci): install Node.js 20 for actions/checkout@v4 compatibility

The default apt nodejs package is too old to support static class blocks
required by checkout@v4. Use NodeSource to install Node.js 20.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/build-package.yaml

@@ -12,9 +12,11 @@ jobs:
     timeout-minutes: 15
 
     steps:
-      - name: Install Node.js (required for actions)
+      - name: Install Node.js 20 (required for actions)
         run: |
-          apt-get update && apt-get install -y nodejs git
+          apt-get update && apt-get install -y curl git
+          curl -fsSL https://deb.nodesource.com/setup_20.x | bash -
+          apt-get install -y nodejs
 
       - name: Checkout repository
         uses: actions/checkout@v4