Move remaining `undef` use in OS extensions to `prepend` #696

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Run time

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Job	Run time
determine-runners	13s
macOS 15-arm64	17s
macOS 15-x86_64	43s
macOS 14-arm64	22s
macOS 14-x86_64	1m 14s
macOS 13-arm64	23s
macOS 13-x86_64	1m 3s
	4m 15s