Update Thread.NativeAot.Unix.cs

Author: jkoritzinsky

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Threading/Thread.NativeAot.Unix.cs

@@ -53,7 +53,7 @@ private static void OnThreadExit()
                 // the thread.
                 currentThread._waitInfo.OnThreadExiting();
                 StopThread(currentThread);
-                SetJoinHandle(currentThread);
+                currentThread.SetJoinHandle();
             }
         }