Pass --proxy Option To Build Environment

[martinezlc99]

Fixes #6018

I have ran into this issue a few times when my corporate proxy was not set in my shell. As mentioned in #6018 (comment), I can reliably replicate this issue in my corporate environment, although I do not make use of any pip environmental variables.

My environment makes use of a private PyPI server and associated extra index URL(s) and trusted host(s). Note in my case, it was only the --proxy option that was missing. I mention this since I cannot say for sure if this will solve all issues, although it seems environmental variables are available in the build environment - hence the workaround mentioned in #6018 (comment).

I did not add any tests as I was not sure how to test the use of a proxy in the build environment. The existing tests for the --proxy option seemed adequate. Glad to add a test for a specific scenario if anyone has any ideas.

[notatallshaw]

I'm going to add this to the 25.0 milestone so it's not forgotten, but someone will still need to volunteer to review and/or accept.