Support use of truststore for pipx install

[Joshua-Schroijen]

How would this feature be useful?

In enterprise environments, TLS interception is commonly used necessitating the use of truststore for downloading packages over HTTPS. That pip package is installed in the same environment as pip runs in and then activated using the line use-feature = truststore in pip.conf/.ini. Without doing so, the user will face TLS/SSL verification errors as certifi most of the time doesn't contain the right TLS certificates to support TLS interceptors.

When running pipx install (whether using --system-site-packages) or not, this does create a chicken-and-egg problem: the pip inside of the new pipx environment doesn't have the truststore package and so can't install anything - including the truststore package. This chicken-and-egg problem also causes the --preinstall option to be useless here.

Please see this discussion thread for background.

Describe the solution you'd like

When building the shared libraries, provide that pip with access to (or use it to install a wheel in the fresh virtual environment of a) recent version of the truststore package. This behavior could be opted into with a command line switch.

Describe alternatives you've considered
I can work around this issue by manually invoking the pip in the shared libraries to install truststore. You can find more details in the discussion thread I had here.