Limit Build workflow to main branch, Fix duplicate runs on PRs

[Copilot]

The Build workflow was firing on every push and every pull request, which caused it to run twice for the same PR branch updates. This narrows the trigger scope so CI runs only for main pushes and main pull requests.

• Workflow trigger scope

  • Restrict push to main
  • Restrict pull_request to main

• Result

  • Prevents duplicate Build executions on non-main PR branches
  • Keeps Build aligned with the branch that actually drives release-ready CI

on:
  push:
    branches: [ main ]
  pull_request:
    branches: [ main ]