: process: fix cleanup race; avoid unwrap

Summary: postfix [this review comment](https://www.internalfb.com/diff/D80022315?dst_version_fbid=791689237351474&transaction_fbid=2649592828720268) from D80022315 - avoid `unwrap()` when pid retrieval fails

Differential Revision: D80262763

Author: shayne-fletcher

hyperactor_mesh/src/alloc/process.rs

@@ -180,7 +180,7 @@ impl Child {
             stderr_tee,
         );
 
-        let process_pid = Arc::new(std::sync::Mutex::new(Some(process.id().unwrap() as i32)));
+        let process_pid = Arc::new(std::sync::Mutex::new(process.id().map(|id| id as i32)));
 
         let child = Self {
             local_rank,