Make the junction crate dependency Windows-only (#3043)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Since the [`junction` crate](https://crates.io/crates/junction)
implements Windows-only functionality, and since the only place it is
used is guarded by `#[cfg(windows)]`,


https://github.com/astral-sh/uv/blob/1f626bfc7300a700257348e1890af90837c740e6/crates/uv-fs/src/lib.rs#L65-L86

it makes sense not to depend on this crate at all on non-Windows
platforms.

If nothing else, this makes Linux distribution packagers’ lives just a
*tiny* bit easier.

## Test Plan

<!-- How was it tested? -->

On Fedora Linux 39:

```
# To avoid an error when /tmp and the working directory are on different filesystems:
$ mkdir _tmp
$ TMPDIR="${PWD}/tmp" cargo run -p uv-dev -- fetch-python
$ cargo test
```

I don’t have access to a Windows system.

crates/uv-fs/Cargo.toml

@@ -20,7 +20,6 @@ dunce = { workspace = true }
 encoding_rs_io = { workspace = true }
 fs-err = { workspace = true }
 fs2 = { workspace = true }
-junction = { workspace = true }
 once_cell = { workspace = true }
 path-absolutize = { workspace = true }
 tempfile = { workspace = true }
@@ -28,6 +27,9 @@ tokio = { workspace = true, optional = true }
 tracing = { workspace = true }
 urlencoding = { workspace = true }
 
+[target.'cfg(windows)'.dependencies]
+junction = { workspace = true }
+
 [features]
 default = []
 tokio = ["dep:tokio", "fs-err/tokio", "backoff/tokio"]