If CMAKE_INSTALL_BINDIR is a different absolute path per project, as
it is with NixOS when we install every package to its own prefix, the
old way fails when the absolute path gets prepended with CMAKE_INSTALL_PREFIX.
The extend_path function does what we want, but it is currently internal-only. So easier to just inline the one small case of it we need.
Also fix one stray bin -> CMAKE_INSTALL_BINDIR