CI: run macos-15-intel only on main (not in PRs)

[marxin]

Depot does not provide macOS Intel runners, and so let's demote them as something we'll be testing only on main branch (or a release branch).

[Copilot]

Pull request overview

This PR optimizes CI costs by configuring the macos-15-intel runner (macos-x64) to skip execution in pull requests and only run on the main branch or release branches. This is achieved by adding a run_in_pr flag to each build metadata configuration and implementing a conditional check at the job level.

Key changes:

• Added run_in_pr boolean field to all metadata entries in the build job matrix
• Set run_in_pr: false for macos-x64 (macos-15-intel) to skip it in PRs
• Set run_in_pr: true for all other platforms (linux-x64, macos-arm, windows-x64, windows-gnu, linux-musl)
• Added job-level conditional to respect the run_in_pr flag and run all builds on non-PR events

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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