runner: do not return 500 if main software release is not built yet

This can happen when the user "opens a software releases" for the first
time : in that case an instance is requested by the runner for this
software release, but the SR is not buit yet (only requested)

slapos/runner/views.py

@@ -197,6 +197,7 @@ def inspectInstance():
   ]
   if current_sr in available_sr_list:
     available_sr_list.remove(current_sr)
+  if current_sr in built_available_sr_list:
     built_available_sr_list.remove(current_sr)
   if "application/json" in request.accept_mimetypes.best:
     result_list = []