Add WASI SDK checksum to CONTRIBUTING.md #1225

Job	Run time
Build for embedded target (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-...	51s
Build for native target (macos-14, Xcode_15.2)	19s
Build and Test (macos-14, wasm-5.9.1-RELEASE, Node, Xcode_15.2.app)	2m 34s
Build for native target (macos-15, Xcode_16)	14s
Build and Test (ubuntu-22.04, wasm-5.9.1-RELEASE, Node)	1m 57s
Build and Test (ubuntu-22.04, wasm-5.10.0-RELEASE, Node)	2m 8s
Build and Test (ubuntu-20.04, wasm-5.10.0-RELEASE, Node)	1m 44s
Build and Test (ubuntu-20.04, wasm-5.9.1-RELEASE, MicroWASI)	2m 10s
Build and Test (ubuntu-20.04, wasm-5.10.0-RELEASE, MicroWASI)	1m 35s
Build and Test (ubuntu-22.04, https://download.swift.org/swift-6.0.2-release/ubuntu2204/swift-6.0...	2m 4s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	2m 27s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	2m 33s
	20m 36s

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