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](https://discord.com/channels/1039017663004942429/1278430431204741270/1278478941262188595)

crates/uv-python/src/environment.rs

@@ -189,22 +189,23 @@ impl PythonEnvironment {
     }
 
     /// Grab a file lock for the environment to prevent concurrent writes across processes.
-    pub fn lock(&self) -> Result<LockedFile, std::io::Error> {
+    pub async fn lock(&self) -> Result<LockedFile, std::io::Error> {
         if let Some(target) = self.0.interpreter.target() {
             // If we're installing into a `--target`, use a target-specific lockfile.
-            LockedFile::acquire(target.root().join(".lock"), target.root().user_display())
+            LockedFile::acquire(target.root().join(".lock"), target.root().user_display()).await
         } else if let Some(prefix) = self.0.interpreter.prefix() {
             // Likewise, if we're installing into a `--prefix`, use a prefix-specific lockfile.
-            LockedFile::acquire(prefix.root().join(".lock"), prefix.root().user_display())
+            LockedFile::acquire(prefix.root().join(".lock"), prefix.root().user_display()).await
         } else if self.0.interpreter.is_virtualenv() {
             // If the environment a virtualenv, use a virtualenv-specific lockfile.
-            LockedFile::acquire(self.0.root.join(".lock"), self.0.root.user_display())
+            LockedFile::acquire(self.0.root.join(".lock"), self.0.root.user_display()).await
         } else {
             // Otherwise, use a global lockfile.
             LockedFile::acquire(
                 env::temp_dir().join(format!("uv-{}.lock", cache_key::cache_digest(&self.0.root))),
                 self.0.root.user_display(),
             )
+            .await
         }
     }