Simplifies use of environment variables

[wpfl-dbt]

Context

We're accruing lots of confusing env files and it's causing conflict.

The core principle of this change is there should only be one .env.example in the repo, which would provide sensible defaults for us to use as we write environment configurations elsewhere. Environments should be configured and stored in the place they’re needed. There is no inheritance of variables.

Changes proposed in this pull request

• The environment variables for integration tests are set in the workflows
• env_file is removed from docker-compose as .env is loaded anyway, and each deployment/testing location ensures the right vars are in the environment
• The environment variables for local unit tests are stored in pyproject.toml and used with pytest-env
• User preferences and local settings (the same thing now) are in .env, copied and edited from .env.example

Guidance to review

• Run every single make test-* command and confirm they all work
• Ensure you're happy with the env variables I've set in docker-compose
• Ensure you're happy with the env variables I've set in your pyproject.toml

Relevant links

• Defining variables in a workflow
• pytest-env
• Docker compose environment variables

Things to check

• I have added any new ENV vars in all deployed environments
• I have tested any code added or changed
• I have run integration tests

[wpfl-dbt]

Closing this PR as it's just not going to make it in and spike/envs still exists if we want to go down that route.