Fix an issue where cache plugins weren't updated

The first call to persisting facts would work due to the assignment of a
MutableMapping calling __setitem__ but subsequent module fact data would
not be propogated to the fact cache plugins because update() doesn't
invoke __setitem__.  This changes the behavior a little bit and ensures
set() is called on cache plugins.

lib/ansible/vars/__init__.py

@@ -546,7 +546,7 @@ class VariableManager:
             self._fact_cache[host.name] = facts
         else:
             try:
-                self._fact_cache[host.name].update(facts)
+                self._fact_cache.update(host.name, facts)
             except KeyError:
                 self._fact_cache[host.name] = facts