Avoid deadlocks when multiple uv processes lock resources (#6790)

This is achieved by updating the `LockedFile::acquire` API to be async —
as in some cases we were attempting to acquire the lock synchronously,
i.e., without yielding, which blocked the runtime.

Closes https://github.com/astral-sh/uv/issues/6691 — I tested with the
reproduction there and a local release build and no longer reproduce the
deadlock with these changes.

Some additional context in the [internal Discord
thread](1278478941)

crates/uv-git/src/resolver.rs

@@ -67,7 +67,8 @@ impl GitResolver {
         let _lock = LockedFile::acquire(
             lock_dir.join(cache_key::cache_digest(&repository_url)),
             &repository_url,
-        )?;
+        )
+        .await?;
 
         // Fetch the Git repository.
         let source = if let Some(reporter) = reporter {