Support environment variables in index URLs in requirements files

[charliermarsh]

Summary

This also preserves the environment variables in the output file, e.g.:

Resolved 1 package in 216ms
# This file was autogenerated by uv via the following command:
#    uv pip compile requirements.in --emit-index-url
--index-url https://test.pypi.org/${SUFFIX}

requests==2.5.4.1

I'm torn on whether that's correct or undesirable here.

Closes #2035.