That’s correct. You need to manually copy the mod-script-pipe directory into …/build/lib-src/
I made a clean build today with the current 2.3.1 alpha code, and found that …/lib-src/mod-script-pipe/ was created automatically.
Note that 2.3.1 alpha is currently NOT suitable for production work as there are problems with several of the built-in effects (see this post: Macros: Change Pitch intermittently zero'd out - #14 by steve)