How to insert variables in Supabase database migrations

Question

I am using a local Supabase CLI to develop my database structure to then deploy to my Supabase project using migrations.

In one of the SQL functions I need to provide the service key and the project URL:

declare
  project_url text := '<YOURPROJECTURL>';
  service_role_key text := '<YOURSERVICEROLEKEY>'; --  full access needed

This is an issue for two reasons:

1. The project url and service role keys differ between my local and online databases, so I would have to manually update these before deploying my changes.
2. These migration files will be uploaded to GitHub, and I don't want to share my service role key.

Is there a way to insert these values as variables in the .sql migration files instead?

From my search I haven't found a good way to inject these variables. Is this possible at all, or what best practice can I follow to resolve this issue?