[Tooling] Run Danger & SwiftLint on the Linter Agent

[iangmaia]

This PR uses the new Linter Agent on Buildkite to run the Danger and SwiftLint jobs.

Because Danger will now run on Buildkite, this PR uses a new GitHub Actions workflow (see Automattic/dangermattic#64 for more details) to retry the Buildkite jobs when the PR state (such as the milestone and labels) changes.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23073-420e099
	Version	24.8
	Bundle ID	org.wordpress.alpha
	Commit	420e099
	App Center Build	WPiOS - One-Offs #9828

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23073-420e099
	Version	24.8
	Bundle ID	com.jetpack.alpha
	Commit	420e099
	App Center Build	jetpack-installable-builds #8876

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.