Change engine/.env to be committed and have .env.local override (#6150)

# Description of Changes
We keep adding stuff to `engine/config/.env.example` and have to
manually update `.env` because of it, which is really clunky, especially
when working on multiple worktrees at once. This PR changes it so that
we just have a committed `.env` file and have an `.env.local` override
to put the actual private keys into, which should make it a bit easier
to manage.

> [!warning]
>
> After this goes in, be very careful for a little while not to
accidentally commit any keys that you've got inside your `.env` file!

.gitignore

@@ -165,6 +165,7 @@ __pycache__/
 # Virtual environments
 .env*
 !.env*.example
+!engine/.env
 .venv*
 env*/
 venv*/