Downgrade "unknown child" (#410)

Summary:
Pull Request resolved: #410

Getting spammed with a lot of these

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: amirafzali

Differential Revision: D77390749

fbshipit-source-id: f262e9bf0436a62420bc279c91069a63fbc496dc

Author: LucasLLC

hyperactor/src/proc.rs

@@ -1065,7 +1065,7 @@ impl<A: Actor> Instance<A> {
                             if let Some(child) = result {
                                 self.cell.unlink(&child);
                             } else {
-                                tracing::warn!("received signal for unknown child pid {}", pid);
+                                tracing::debug!("received signal for unknown child pid {}", pid);
                             }
                         },
                     }