Use the play term instead of playbook.

An ansible playbook is a list of plays.  environment: applies per-play,
not per-playbook.

docsite/rst/playbooks_environment.rst

@@ -33,7 +33,7 @@ The environment can also be stored in a variable, and accessed like so::
         - apt: name=cobbler state=installed
           environment: "{{proxy_env}}"
 
-You can also use it at a playbook level::
+You can also use it at a play level::
 
     - hosts: testhost