Add merge queue note to Copilot setup steps example

[ArthurFay]

Summary

Updates the copilot-setup-steps.yml example to avoid unnecessary runs in merge queues.

Changes made

• added branches-ignore for gh-readonly-queue/**
• limited pull_request to main

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-environment.md	fpt ghec	fpt ghec
actions/how-tos/manage-workflow-runs/approve-runs-from-forks.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	from reusable

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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