Avoid race when unpacking wheels (#593)

## Summary

If someone else beats us to the unzip, we should let them win.

We already have a check for this at the top of the unzip method, but
it's also possible that two source distributions get built in parallel
that both try to unpack the same build dependency.

crates/puffin-installer/src/unzipper.rs

@@ -59,7 +59,15 @@ impl Unzipper {
                     download.unzip(staging.path())?;
 
                     // Move the unzipped wheel into the cache,.
-                    fs_err::rename(staging.into_path(), download.target())?;
+                    if let Err(err) = fs_err::rename(staging.into_path(), download.target()) {
+                        // If another thread already unpacked the wheel, we can ignore the error.
+                        return if download.target().is_dir() {
+                            warn!("Wheel is already unpacked: {}", download.remote());
+                            Ok(download.target().to_path_buf())
+                        } else {
+                            Err(err.into())
+                        };
+                    }
 
                     Ok(download.target().to_path_buf())
                 }