Fix `no-use-before-define` lint violation in `environment.d.ts`

[bmish]

Running linting in a brand new Ember addon with TypeScript with the ESLint no-use-before-define lint rule enabled:

~/Development/my-test-addon main * yarn eslint --ext=.ts,.js .

~/Development/my-test-addon/tests/dummy/app/config/environment.d.ts
  1:16  error  'config' was used before it was defined                                         no-use-before-define

This is an easy and harmless fix to avoid this lint violation for anyone using this lint rule.