pythonPackages.pytorch: build fails to build on NixOS unstable

[dnaq]

Issue description

When trying to build pytorch the following error occurs.

Traceback (most recent call last):
  File "nix_run_setup", line 8, in <module>
    exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\\r\\n', '\\n'), __file__, 'exec'))
  File "setup.py", line 170, in <module>
    from tools.setup_helpers.cuda import USE_CUDA, CUDA_HOME, CUDA_VERSION
  File "/build/source/tools/setup_helpers/cuda.py", line 75, in <module>
    cuda_path = find_nvcc()
  File "/build/source/tools/setup_helpers/cuda.py", line 17, in find_nvcc
    proc = Popen(['which', 'nvcc'], stdout=PIPE, stderr=PIPE)
  File "/nix/store/gbwx5sc4w0i0ipnmsa0nsk88svs983vl-python3-3.7.2/lib/python3.7/subprocess.py", line 775, in __init__
    restore_signals, start_new_session)
  File "/nix/store/gbwx5sc4w0i0ipnmsa0nsk88svs983vl-python3-3.7.2/lib/python3.7/subprocess.py", line 1522, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'which': 'which'
builder for '/nix/store/nc8x0hqhw1739z4i0zb9yxq4rg0lylk1-python3.7-pytorch-1.0.0.drv' failed with exit code 1
cannot build derivation '/nix/store/s246vil3pa9lfwjdwdkz6ai5gy41p64d-python3-3.7.2-env.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/c3jgz96yl9nmk6c536pjig214mizjz0x-system-path.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/s1n9d9p77cp8qwbafb4dzrq20jigw5mv-nixos-system-nixos-19.09pre171786.34aa254f9eb.drv': 1 dependencies couldn't be built
error: build of '/nix/store/s1n9d9p77cp8qwbafb4dzrq20jigw5mv-nixos-system-nixos-19.09pre171786.34aa254f9eb.drv' failed

This seems to be related to #54182, seems like buildInputs is now ignored when building python modules.

Steps to reproduce

Try to build pytorch, e.g. nix-build -A python3Packages.pytorch

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.103, NixOS, 19.09pre170705.1233c8d9e9b (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.2
• channels(root): "nixos-19.09pre171786.34aa254f9eb"
• channels(user) ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

[dnaq]

pinging the package maintainers @teh and @thoughtpolice