_scheduler/queues: Mark build and pull queue as requiring shared access to the CACHE

This is what the whole resource.py thing was created for, the
cleanup job must have exclusive access to the cache, while the pull
and build jobs which result in adding new artifacts, must only require
shared access.

This is a part of #623

buildstream/_scheduler/queues/buildqueue.py

@@ -32,7 +32,7 @@ class BuildQueue(Queue):
 
     action_name = "Build"
     complete_name = "Built"
-    resources = [ResourceType.PROCESS]
+    resources = [ResourceType.PROCESS, ResourceType.CACHE]
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)

buildstream/_scheduler/queues/pullqueue.py

@@ -29,7 +29,7 @@ class PullQueue(Queue):
 
     action_name = "Pull"
     complete_name = "Pulled"
-    resources = [ResourceType.DOWNLOAD]
+    resources = [ResourceType.DOWNLOAD, ResourceType.CACHE]
 
     def process(self, element):
         # returns whether an artifact was downloaded or not