timer: Assign the loop _before_ running the process

Avoid a potential race condition. Assign the loop variable before
starting the process. Otherwise the effect on loop could be that a
terminated process is being assigned.

Spotted while reviewing a problem Norbert experienced in Pharo

Timer.st

@@ -107,9 +107,12 @@ bit difficult to do this race free.'>
         <category: 'creation'>
 
         processExited := Semaphore new.
-        loop := [[Processor activeProcess name: 'Osmo Timers'.
-                self runTimers
-                ] ensure: [processExited signal. loop := nil]] fork
+
+        loop := [[self runTimers] ensure: [
+                    processExited signal.
+                    loop := nil]] newProcess.
+        loop name: 'Osmo Timers'.
+        loop resume.
     ]
 
     scheduleInSeconds: aDelay block: aBlock [