Pip.conf settings are not parsed or respected

An example pip.conf with sensitive variables changed to look like BASH variables:

```ini
[global]
extra-index-url = https://aws:${CODEARTIFACT_AUTH_TOKEN}@${DOMAIN}-${ACCOUNT_ID}.d.codeartifact.us-east-1.amazonaws.com/pypi/public-pypi/simple/
index-url = https://aws:${CODEARTIFACT_AUTH_TOKEN}@${DOMAIN}-${ACCOUNT_ID}.d.codeartifact.us-east-1.amazonaws.com/pypi/private-releases/simple/
```
These are actually set using the `pip config set global.extra-index-url` so they are presumably following the correct convention.

The file is located at `$HOME/.config/pip/pip.conf` following the [XDG_CONFIG_HOME](https://pip.pypa.io/en/stable/topics/configuration/#location) even when it is located in the legacy location `$HOME/.pip/pip.conf` it is not respected either.

The [pip documentation](https://pip.pypa.io/en/stable/topics/configuration/) in general.

EDIT: It also does not respect the relevant [environment variables either](https://pip.pypa.io/en/stable/topics/configuration/#environment-variables)