Don't use listify_ function, when all we want to do is template variables

This was causing a bug in the env module, due to the fact that we now
pass variables for the module through the templating engine combined
with the fact that we split-up the hostvars and setup variables. As a
result, if a variable in the env lookup had the same name as the variable
in Ansible, it would try and template itself over and over again until
the recursion limit would be hit, at which time an empty string was
returned.

Fixes #7396
Merge pull request #7481 from jimi-c/issue_7396_env_lookup

Don't use listify_ function, when all we want to do is template variable...