[Build Script Helper] Build with a specified toolchain improvements     - Only set environment variables for executables built by `swift` build-script as a part of a toolchain.     - Export SDKROOT on Darwin to ensure tests always have an SDK if one is not specified explicitly.
[Build Script Helper] Build with a specified toolchain improvements
    - Only set environment variables for executables built by `swift` build-script as a part of a toolchain.
    - Export SDKROOT on Darwin to ensure tests always have an SDK if one is not specified explicitly.

apple/swift-driver,apple/swift-driver,apple/swift-driver,apple/swift-driver