Unset PIP_REQUIRE_VIRTUALENV when running 'pip'

[adamchainz]

This is the same issue as in pep517: pypa/pyproject-hooks#84

If PIP_REQUIRE_VIRTUALENV is set (to a non-empty value) when running python -m build , pip errors with its "ERROR: Could not find an activated virtualenv (required)." message:

$ python -m build
Looking in links: /var/folders/kq/02p9h16n7zv_z7bhzmg6y_pm0000gn/T/tmpo1xlgb08
Processing /private/var/folders/kq/02p9h16n7zv_z7bhzmg6y_pm0000gn/T/tmpo1xlgb08/setuptools-47.1.0-py3-none-any.whl
Processing /private/var/folders/kq/02p9h16n7zv_z7bhzmg6y_pm0000gn/T/tmpo1xlgb08/pip-20.1.1-py2.py3-none-any.whl
Installing collected packages: setuptools, pip
Successfully installed pip setuptools
ERROR: Could not find an activated virtualenv (required).
Traceback (most recent call last):
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/site-packages/build/__main__.py", line 176, in <module>
    main(sys.argv[1:], 'python -m build')
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/site-packages/build/__main__.py", line 168, in main
    build(args.srcdir, args.outdir, distributions, config_settings, not args.no_isolation, args.skip_dependencies)
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/site-packages/build/__main__.py", line 80, in build
    _build_in_isolated_env(builder, outdir, distributions)
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/site-packages/build/__main__.py", line 45, in _build_in_isolated_env
    env.install(builder.build_dependencies)
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/site-packages/build/env.py", line 176, in install
    subprocess.check_call(cmd)
  File "/Users/chainz/.pyenv/versions/3.8.5/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/Users/chainz/.pyenv/versions/3.8.5/bin/python', '-m', 'pip', 'install', '--prefix', '/var/folders/kq/02p9h16n7zv_z7bhzmg6y_pm0000gn/T/build-env-9j1lejv3', '-r', '/var/folders/kq/02p9h16n7zv_z7bhzmg6y_pm0000gn/T/build-reqs-cc8vednw.txt']' returned non-zero exit status 3.

The solution is to unset the environment variable for the subprocess.