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Fix path-resolution bug in `changenotes.mts validate` by using `getChangenotes` #9043

Job	Run time
Ruby analysis (macos-latest, default)	1m 6s
Ruby analysis (ubuntu-latest, default)	44s
Ruby analysis (macos-latest, nightly-latest)	1m 11s
Ruby analysis (ubuntu-latest, nightly-latest)	44s
Ruby analysis (ubuntu-latest, linked)	38s
Ruby analysis (macos-latest, linked)	58s
	5m 21s