Drop the special case for the "build-tools" environment

[ncoghlan]

If a "build-tools" virtual environment exists inside the build folder, venvstacks will use that instead of the interpreter running venvstacks to run uv and pip.

This is a legacy of the early prototyping phase before venstacks became a full-fledged Python project in its own right, and it isn't necessary any more (these days, venvstacks itself is installed into the "build-tools" environment, so the special case ends up referring back to sys.executable anyway).

This will be dropped as part of #64

[ncoghlan]

Resolved in #64