_scheduler/jobs/cleanupjob.py: Use Platform.get_platform() public accessor

This was previously poking directly at the Platform._instance.

Also, use the name 'artifacts' to hold the artifact cache to
be consistent with other parts of the codebase, instead of
calling it 'cache'.

buildstream/_scheduler/jobs/cleanupjob.py

@@ -24,13 +24,15 @@ class CleanupJob(Job):
     def __init__(self, *args, complete_cb, **kwargs):
         super().__init__(*args, **kwargs)
         self._complete_cb = complete_cb
-        self._cache = Platform._instance.artifactcache
+
+        platform = Platform.get_platform()
+        self._artifacts = platform.artifactcache
 
     def child_process(self):
-        return self._cache.clean()
+        return self._artifacts.clean()
 
     def parent_complete(self, success, result):
-        self._cache.set_cache_size(result)
+        self._artifacts.set_cache_size(result)
         if self._complete_cb:
             self._complete_cb()