pipenv commands are broken on Windows: 'env' is not recognized

[hroncok]

> pipenv run naucse serve
Creating a virtualenv for this project…
Pipfile: C:\...\naucse.python.cz\Pipfile
Using C:/.../Python/Python36/python.exe (3.6.8) to create virtualenv…
[ ===] Creating virtual environment...Using base prefix 'C:\\...\\Python\\Python36'
New python executable in C:\...\.virtualenvs\naucse.python.cz-GmZHW9w3\Scripts\python.exe
Installing setuptools, pip, wheel...
done.
Running virtualenv with interpreter C:/.../Python/Python36/python.exe

Successfully created virtual environment!
Virtualenv location: C:\...\.virtualenvs\naucse.python.cz-GmZHW9w3
'env' is not recognized as an internal or external command,
operable program or batch file.

The reason for this is that our commands are:

naucse.python.cz/Pipfile

Lines 6 to 10 in 67e523b

	[scripts]
	naucse = "env PYTHONPATH=. python -m naucse"
	serve = "env PYTHONPATH=. python -m naucse serve"
	freeze = "env PYTHONPATH=. python -m naucse freeze"
	test = "python -m pytest test_naucse/"

AFAIK env doesn't work as a command on Windows, but I seriously don't know.

[hroncok]

(This blocks @Zuzanita when trying to assemble the new Prague runs of PyLadies beginner courses.)

[hroncok]

Note that without env, it causes (on Linux):

Error: the command PYTHONPATH=. (from serve) could not be found within PATH.