[Infra] Only compile core extension headers once #3111

Job	Run time
spm-package-resolved	59s
pod-lib-lint (ios, macos-14, Xcode_15.2)	3m 55s
pod-lib-lint (ios, macos-14, Xcode_16)	3m 19s
pod-lib-lint (tvos, macos-14, Xcode_15.2)	3m 17s
pod-lib-lint (tvos, macos-14, Xcode_16)	4m 3s
pod-lib-lint (macos, macos-14, Xcode_15.2)	2m 7s
pod-lib-lint (macos, macos-14, Xcode_16)	6m 18s
pod-lib-lint (watchos, macos-14, Xcode_15.2)	2m 50s
pod-lib-lint (watchos, macos-14, Xcode_16)	2m 31s
spm-integration (macos-14, Xcode_15.4)	22m 54s
spm-unit (macos-13, Xcode_15.2, iOS)	7m 24s
spm-unit (macos-14, Xcode_15.4, iOS)	7m 19s
spm-unit (macos-15, Xcode_16, iOS)	1m 57s
spm-unit (macos-15, Xcode_16, tvOS)	1m 55s
spm-unit (macos-15, Xcode_16, macOS)	1m 39s
spm-unit (macos-15, Xcode_16, watchOS)	1m 51s
spm-unit (macos-15, Xcode_16, catalyst)	1m 45s
spm-unit (macos-15, Xcode_16, visionOS)	1m 54s
functions-cron-only	0s
	1h 17m 57s

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