_scheduler/scheduler.py: Ignore interrupt events while terminating.

For some reason, we now receive a SIGINT from the main loop even
when the SIGINT occurred with the handler disconnected in an interactive
prompt. This patch simply ignores any received SIGINT events from the
main loop in the case that we are already in the process of terminating.

This fixes issue #693

buildstream/_scheduler/scheduler.py

@@ -387,6 +387,15 @@ class Scheduler():
     # A loop registered event callback for keyboard interrupts
     #
     def _interrupt_event(self):
+
+        # FIXME: This should not be needed, but for some reason we receive an
+        #        additional SIGINT event when the user hits ^C a second time
+        #        to inform us that they really intend to terminate; even though
+        #        we have disconnected our handlers at this time.
+        #
+        if self.terminated:
+            return
+
         # Leave this to the frontend to decide, if no
         # interrrupt callback was specified, then just terminate.
         if self._interrupt_callback: