Move to async implementations of `UserToolchain.init` and `SwiftSDK.hostSwiftSDK` #69

Job	Run time
Test / macOS (Xcode 26.0 - sequoia - ARM64)	3m 17s
Test / iOS (Build Only, Xcode 26.0 - sequoia - ARM64)	4m 0s
Test / Linux (nightly-main - amazonlinux2)	10m 12s
Test / Linux (nightly-main - bookworm)	9m 15s
Test / Linux (nightly-main - rhel-ubi9)	10m 52s
Test / Windows (nightly-main - windows-2022)	35m 24s
Test / Linux (nightly-main - jammy)	9m 28s
Test / Linux (nightly-main - noble)	9m 48s
Test / Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test / Embedded Swift SDK for Wasm Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Test / Swift SDK for Android Build (${{ matrix.swift_version }} - ${{ matrix.os_version }} - NDK ${{ matrix.ndk_version }})	0s
Test / Static Linux Swift SDK Build (${{ matrix.swift_version }} - ${{ matrix.os_version }})	0s
Soundness / Python lint check	10s
Soundness / Documentation check	46s
Soundness / Broken symlinks check	8s
Soundness / License headers check	0s
Soundness / YAML lint check	6s
Soundness / Unacceptable language check	0s
Soundness / API breakage check	0s
Soundness / Format check	0s
Soundness / Shell check	0s
	1h 33m 26s

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