[gentoo-devhelp] Cannot get build directory in multilib ebuild

[Nikos Chantziaras]

I need to install a file that the package generates in its build 
directory. It's a cmake project. I tried:

   doins "${BUILD_DIR}/some_file"

But it doesn't find the file:

   FileNotFoundError: [Errno 2] No such file or directory: b'/some_file'

The ebuild inherits cmake-multilib. There's two build directories of 
course, since this is a multilib ebuild:

   package-version-abi_x86_32.x86
   package-version-abi_x86_64.amd64


I'm interested in the native build directory (the amd64 one in this 
case.) Is there a variable somewhere that I'm just not finding, or do I 
have to construct the build directory path myself?