Fixing new caching related issue with host vars

The vars_cache was not being properly merged with the setup_cache
for all hosts, which was previously not noticed when registered
variables were stored in the setup_cache.

Fixes #8944

lib/ansible/runner/__init__.py

@@ -598,7 +598,7 @@ class Runner(object):
 
         # merge the VARS and SETUP caches for this host
         combined_cache = self.setup_cache.copy()
-        combined_cache.setdefault(host, {}).update(self.vars_cache.get(host, {}))
+        combined_cache.update(self.vars_cache)
 
         hostvars = HostVars(combined_cache, self.inventory, vault_password=self.vault_pass)