discordapp.com domain will go defunct very soon, everything has transitioned to discord.com now

https://support.discord.com/hc/en-us/articles/360042987951-Discordapp-com-is-now-Discord-com

Replace updates of os.environ with explcit passing of `env` to
subprocess calls.

Relates-to: #3199

Resolves: #3303

This change allows users to run embedded pip via `poetry run pip`.

Use embedded pip wheel from virtualenv package. This avoids the need
for pip to be a critical package in the project's virtual environment.

In some cases, legacy repositories might not provide a checksum as a
url fragment or use a deprecated algorithm. In these scenarios, this
change ensures that poetry downloads and calculates the sha256 checksum
for the file.

Resolves: #1631 #1553