Switch to Current-Thread Tokio Runtime (#4934)

## Summary

Move completely off tokio's multi-threaded runtime. We've slowly been
making changes to be smarter about scheduling in various places instead
of depending on tokio's general purpose work-stealing, notably
https://github.com/astral-sh/uv/pull/3627 and
https://github.com/astral-sh/uv/pull/4004. We now no longer benefit from
the multi-threaded runtime, as we run on all I/O on the main thread.
There's one remaining instance of `block_in_place` that can be swapped
for `rayon::spawn`.

This change is a small performance improvement due to removing some
unnecessary overhead of the multi-threaded runtime (e.g. spawning
threads), but nothing major. It also removes some noise from profiles.

## Test Plan

```
Benchmark 1: ./target/profiling/uv (resolve-warm)
  Time (mean ± σ):      14.9 ms ±   0.3 ms    [User: 3.0 ms, System: 17.3 ms]
  Range (min … max):    14.1 ms …  15.8 ms    169 runs
 
Benchmark 2: ./target/profiling/baseline (resolve-warm)
  Time (mean ± σ):      16.1 ms ±   0.3 ms    [User: 3.9 ms, System: 18.7 ms]
  Range (min … max):    15.1 ms …  17.3 ms    162 runs
 
Summary
  ./target/profiling/uv (resolve-warm) ran
    1.08 ± 0.03 times faster than ./target/profiling/baseline (resolve-warm)
```

crates/uv-dispatch/src/lib.rs

@@ -278,16 +278,17 @@ impl<'a> BuildContext for BuildDispatch<'a> {
         }
 
         // Install the resolved distributions.
-        let wheels = wheels.into_iter().chain(cached).collect::<Vec<_>>();
+        let mut wheels = wheels.into_iter().chain(cached).collect::<Vec<_>>();
         if !wheels.is_empty() {
             debug!(
                 "Installing build requirement{}: {}",
                 if wheels.len() == 1 { "" } else { "s" },
                 wheels.iter().map(ToString::to_string).join(", ")
             );
-            Installer::new(venv)
+            wheels = Installer::new(venv)
                 .with_link_mode(self.link_mode)
-                .install(&wheels)
+                .install(wheels)
+                .await
                 .context("Failed to install build dependencies")?;
         }