Use Node.js 20.12.2 instead of 'latest'

Attempt to work around nodejs/node#52708 /
gulpjs/vinyl-fs#350 by reverting to previous
LTS version.

Author: oddhack

.github/workflows/ci.yml

@@ -68,7 +68,7 @@ jobs:
       - name: "setup npm"
         uses: actions/setup-node@v4
         with:
-          node-version: 'latest'
+          node-version: '20.12.2'
           cache: 'npm'
           cache-dependency-path: docs-site/package-lock.json