[CI] Change AzDO public pipeline to run weekly instead of per-commit

[Copilot]

This PR changes the Azure DevOps public pipeline to run on a weekly schedule rather than on every commit to the main branch, as requested in the issue.

Changes

• Modified eng/pipelines/azure-pipelines-public.yml to:
  • Replace the continuous integration trigger with a scheduled trigger
  • Configure the pipeline to run at midnight (UTC) every Monday
  • Set always: true to ensure the build runs weekly regardless of code changes
  • Set trigger: none to explicitly disable continuous integration triggers

Benefits

• Reduces CI resource usage as we now have regular rolling builds on GitHub Actions
• Maintains the same branch filtering from the original configuration
• Preserves PR validation to continue testing pull requests

The pipeline will continue to build the same branches (main and release/*) but on a weekly cadence instead of for every commit.

Fixes #9436.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.