Merge module_vars into inject during template instead of updating

This way we won't overwrite values that are stored in the inject
with module_vars that should be lower precedence.

Fixes #7510

lib/ansible/runner/__init__.py

@@ -580,7 +580,7 @@ class Runner(object):
         # we update the inject variables with the data we're about to template
         # since some of the variables we'll be replacing may be contained there too
         module_vars_inject = utils.combine_vars(combined_cache.get(host, {}), host_variables)
-        module_vars_inject.update(self.module_vars)
+        module_vars_inject = utils.combine_vars(self.module_vars, module_vars_inject)
         module_vars = template.template(self.basedir, self.module_vars, module_vars_inject)
 
         inject = {}