[Infra] Only compile core extension headers once #2663

Job	Run time
catalyst	3m 1s
pod-lib-lint (macos-13, Xcode_15.2, ios)	10m 44s
spm-package-resolved	1m 12s
pod-lib-lint (macos-14, Xcode_15.4, ios)	6m 24s
pod-lib-lint (macos-15, Xcode_16, ios)	3m 24s
pod-lib-lint (macos-15, Xcode_16, tvos)	2m 32s
pod-lib-lint (macos-15, Xcode_16, macos)	2m 13s
pod-lib-lint (macos-15, Xcode_16, watchos)	1m 18s
quickstart	11m 56s
quickstart-ftl-cron-only	0s
spm (macos-13, Xcode_15.2, iOS)	4m 42s
spm (macos-14, Xcode_15.4, iOS)	7m 18s
spm (macos-15, Xcode_16, iOS)	1m 56s
spm (macos-15, Xcode_16, tvOS)	2m 21s
spm (macos-15, Xcode_16, macOS)	1m 26s
spm (macos-15, Xcode_16, watchOS)	2m 23s
spm (macos-15, Xcode_16, catalyst)	1m 39s
spm (macos-15, Xcode_16, visionOS)	1m 46s
abtesting-cron-only	0s
	1h 6m 15s

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