poetry install partially broken after introducing --isolated #7182

#6531 added the --isolated flag to the pip invocation. This seems to be breaking installation in certain situations.

Our analysis suggests this is the reason:

• Poetry 1.3 started passing --isolated to pip.
• The new switch results in ignoring all user configurations, like a PIP_INDEX_URL or a user-local pip.conf file.
• Since poetry is not passing the configured index to pip (related to Provide index to pip for installation of sdist build dependencies  #3249?), pip is now called without any index configuration.
• pip wants to build the wheel for the installed package.
• Packages may have additional setup_requires dependencies needed for building them.
• pip tries downloading those. Since there is no index config, pip attempts to use the default, PyPI.
• On machines with without internet access (like enterprise build machines), this fails.

Workaround: Create a non-user configuration, e.g. in /etc/pip.conf.

In a way the new switch stopped shadowing the issue that the index config is not passed to pip, that's why this is surfacing now.

I'd appreciate some feedback how the developer team sees that issue.