SR-6450: Leak in Process on Linux

- The runLoopSource was holding a reference to self creating a retain
  cycle. Set to nil after the child process has finished.

(cherry picked from commit b15e4f4)

Author: spevans

Foundation/Process.swift

@@ -1128,6 +1128,7 @@ open class Process: NSObject {
         } while( self.isRunning == true && RunLoop.current.run(mode: .default, before: Date(timeIntervalSinceNow: 0.05)) )
 
         self.runLoop = nil
+        self.runLoopSource = nil
     }
 }