python: Better host pip options

python: Better host pip options

pip by default will read system-wide and per-user configuration
files[1]. Setting PIP_CONFIG_FILE=/dev/null instructs pip to not read
any config files[2].

pip will spawn child processes of itself to do work, but not all options
are passed down to the child processes[3]. Setting global options as
environment variables[4] ensures they are passed down to any child
processes.

[1]: https://pip.pypa.io/en/stable/topics/configuration/#configuration-files
[2]: https://pip.pypa.io/en/stable/topics/configuration/#pip-config-file
[3]: https://github.com/pypa/pip/issues/9081#issue-733819665
[4]: https://pip.pypa.io/en/stable/topics/configuration/#environment-variables

Signed-off-by: Jeffery To <jeffery.to@gmail.com>