MAINT: rework data-find-url behaviour

Rework the order of precedence for data-find-url.

I propose reworking the order to the following:

1. Value from the config - takes the highest priority and overrides all other values
2. The environment variable GWDATAFIND_SERVER
3. The default stored in bilby_pipe.utils called DEFAULT_GWDATAFIND_SERVER

This also changes environment and get-env to follow this same order.

To-Do

• End-to-end tests
• Unit tests
• Update documentation