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 easily to just inline the one small case of it we need. Also fix one stray `bin` -> `CMAKE_INSTALL_BINDIR`

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`

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 easilyer to just inline the one small case of it we need. Also fix one stray `bin` -> `CMAKE_INSTALL_BINDIR`