Gate `ExperimentalCustomExecutors` usage behind Swift 6.4 compiler check #2508

Job	Run time
format	31s
check-bridgejs-generated	2m 34s
build-examples	19m 42s
Test BridgeJS against Swift versions (swift:6.2, 602.0.0)	3m 19s
Test BridgeJS against Swift versions (swift:6.1.2, 601.0.0)	3m 4s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	17m 33s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 21s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	13m 7s
Build for native target (macos-15, Xcode_16.4)	2m 14s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	13m 49s
prettier	9s
deploy-examples	0s
	1h 29m 23s

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