ci: install Node.js and Yarn inside container for metadata-regen

The metadata-regen job runs inside the px4io/px4-dev container which
has no Node.js. The actions/setup-node action only installs on the
host runner, not inside the container, causing yarn: not found errors.

Replace actions/setup-node with direct Node.js installation via
NodeSource and enable corepack for Yarn support.

Signed-off-by: Ramon Roche <mrpollo@gmail.com>

.github/workflows/docs-orchestrator.yml

@@ -178,12 +178,11 @@ jobs:
         env:
           CCACHE_DIR: ~/.ccache
 
-      - name: Setup Node.js
-        uses: actions/setup-node@v4
-        with:
-          node-version: 20
-          cache: npm
-          cache-dependency-path: ./docs/yarn.lock
+      - name: Install Node.js and Yarn
+        run: |
+          curl -fsSL https://deb.nodesource.com/setup_20.x | bash -
+          apt-get install -y nodejs
+          corepack enable
 
       - name: Format markdown with Prettier
         run: |