lock: treat PermissionDenied on Windows as transient error

On Windows it can be PermissionDenied when creating the new file
exclusively. This change makes lock_concurrent test pass on Windows.

lib/src/lock.rs

@@ -36,6 +36,9 @@ impl FileLock {
             Err(err) if err.kind() == std::io::ErrorKind::AlreadyExists => {
                 Err(backoff::Error::Transient(err))
             }
+            Err(err) if cfg!(windows) && err.kind() == std::io::ErrorKind::PermissionDenied => {
+                Err(backoff::Error::Transient(err))
+            }
             Err(err) => Err(backoff::Error::Permanent(err)),
         };
         let mut backoff = ExponentialBackoff {