example.yml

vars.yml
---
env:
  ANSIBLE_LIBRARY: "~/modules"
####
test.yml
  - hosts: all
    vars_files:                                                                 
       - vars.yml                                                               
    environment: "{{env}}"                                                      
    tasks:
       - name: echo env                                                         
       # THIS WORKS
         command: echo $ANSIBLE_LIBRARY # {{ lookup('env, 'ANSIBLE_LIBRARY') }} 
         register: ekko                                                         
                                                                                
       - debug: var=ekko                                                        
                                                                                
       - name: My module                                         
       # BUT THIS FAILS CAUSE IT CANT FIND THE MODULE
         my_module:                                                   
            input: "this-is-a-test"                                            
         register: result                                                       
                                                                                
       - debug: var=result      
###
~/modules/my_module.py holds the external module which works if I set ANSIBLE_LIBRARY through bash