add workflow_dispatch trigger to build-node

This enables the manual "Run workflow" button in the GitHub Actions UI.

Co-Authored-By: Claude (global.anthropic.claude-opus-4-5-20251101-v1:0) <noreply@anthropic.com>

Author: JackStrohm-asana

.github/workflows/build-node.yml

@@ -1,6 +1,7 @@
 name: Build Node
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - v20.18.3