Packages can't be installed in isolated env if pip is in `user' mode

[layday]

If a user has configured pip to run in user mode, it will be unable to install packages in build's isolated environment. See: pypa/packaging-problems#459 (comment).

[FFY00]

How do we override that?

[layday]

I suppose we could set PIP_USER to 0 or pass --no-user to every pip command that we issue, if this is something that we want to handle in build.

[dtrodrigues]

Is --no-user a supported flag? pypa/pip#8977 (comment)

[layday]

build is not an "end user" in this context.

[PaperStrike]

Just to make sure you have noticed the close reason, pypa/pip#8977 (comment) :

pip has real config in a virtual environment, at $(sys.prefix)/pip.conf. Having a user = false there would override the user configuration.

It seems like it should be set on venv or virtualenv side, since no one would like to do a real pip install in virtual envrionment? Or is it better to be set by every module that creates virtual envrionment and wants to install modules inside? What do you think?

[uranusjr]

It’d be pretty weird for the built-in venv to create a tool-specific config file; I’m more inclined to think pip and/or tools using virtual environments should fix this on its own.

Regarding pip, the logic around scheme selection really needs an complete redesign IMO (see discussion in pypa/pip#7829), and --user is only one sympton to the problem. But it has a lot of combinations and needs someone to sit down for a long time and work out all of the possibilities, which is unsurprisingly not a very popular task.