Closes#14262
## Description
Adds `UV_LIBC` environment variable and implements check within
`Libc::from_env` as recommended here:
https://github.com/astral-sh/uv/issues/14262#issuecomment-3014600313
Gave this a few passes to make sure I follow dev practices within uv as
best I am able. Feel free to call out anything that could be improved.
## Test Plan
Planned to simply run existing test suite. Open to adding more tests
once implementation is validated due to my limited Rust experience.
Extends https://github.com/astral-sh/uv/pull/13841 — I'll drop that
commit later after that pull request merges but it's small.
I find the split into a "Configuration" section awkward and don't think
it's helping us. Everything moved into the "Concepts" section, except
the "Environment variables" page which definitely belongs in the
reference and the "Installer" page which is fairly niche and seems
better in the reference.
Before / After
<img
src="https://github.com/user-attachments/assets/80d8304b-17da-4900-a5f4-c3ccac96fcc5"
width="400">
Nathan Cain
Zanie Blue
Jack O'Connor