I've built and activated a custom toolchain. Build logs confirm that the custom swift compiler is being used. However, it doesn't work with the new (preview) build system. Build logs still show the correct "export TOOLCHAINS=..." line, but then it uses swift from "XcodeDefault.xctoolchain".
The only difference I see is that the new build system adds an additional environment variable:
export SWIFTC="Apple Swift version 4.1 (swiftlang-902.0.48 clang-902.0.37.1)
Is it a current limiration of the new build system?