Build: Enforce SWIFT_SDK_ID is set for unittest target #1937

Job	Run time
Build for native target (macos-15, Xcode_16.4)	2m 55s
format	34s
check-bridgejs-generated	2m 47s
build-examples	44m 18s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 0s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.1-release/ubuntu2204/swift-6.1-R...	13m 10s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	13m 2s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	17m 2s
deploy-examples	10s
	1h 46m 58s

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