Make the toolchain work without Xcode installed

[MaxDesiatov]

Seems that swift build assumes xctest is available somewhere in PATH directories when building SwiftWasm projects on macOS as reported by @cohix: https://twitter.com/cohix/status/1320734262958661632?s=21

Using swift build —triple=wasm32-unknown-wasi outputs the error “xcrun: error: unable to find utility "xctest", not a developer tool or in PATH”

(but calling swiftc directly works fine, which I may end up doing)