Add support for validation jobs to `sync.ts`, and refactor #7155

Job	Run time
Swift analysis using a custom build command (macos-latest, default)	6m 50s
Swift analysis using a custom build command (macos-latest, nightly-latest)	8m 15s
Swift analysis using a custom build command (macos-latest, linked)	7m 1s
	22m 6s

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