Use space as delimiter for `UV_EXTRA_INDEX_URL` (#2140)
## Summary
I was looking at something unrelated and saw this in the Clap docs.
Closes https://github.com/astral-sh/uv/issues/1702.
## Test Plan
```shell
❯ UV_EXTRA_INDEX_URL="https://google.com https://foo.com" cargo run pip compile -
Finished dev [unoptimized + debuginfo] target(s) in 0.14s
Running `target/debug/uv pip compile -`
index: None
extra_index_url: [Url(VerbatimUrl { url: Url { scheme: "https", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("google.com")), port: None, path: "/", query: None, fragment: None }, given: Some("https://google.com") }), Url(VerbatimUrl { url: Url { scheme: "https", cannot_be_a_base: false, username: "", password: None, host: Some(Domain("foo.com")), port: None, path: "/", query: None, fragment: None }, given: Some("https://foo.com") })]
```
