fix(789): use system env for header

[c-m-duncan]

Ticket Link #789

Description

There's a race condition in how this compiles, so we can't use the application variable

Reminders:

• If changing elixir code in an "app", did you update the relevant version
  in mix.exs?
• If altering an API endpoint, was the relevant postman collection updated?
  • If a new version of smart_city is being used (new fields on a struct), were the relevant postman collections updated?