cached and generated files are hard coded to be rooted at source location

[toffaletti]

See: https://github.com/apple/swift-sdk-generator/blob/db85e25d97d0443560cfcff848d20f7690868454/Sources/SwiftSDKGenerator/Generator/SwiftSDKGenerator.swift#L40

This means you can't really install swift-sdk-generator into your PATH and have it generate an sdk to an arbitrary location. I would propose using a location in ~/Library/Caches for the toolchain cache and put the output in the current working directory or allow a cli flag to specify.