Don't require ws-worker keys to run migrations in prod

[taylordowns2000]

When running OpenFn in production mode on kubernetes, some folks may choose to automatically run migrations using an initContainer:

initContainers:
        - name: migrate-demo
          image: *demoImage
          command: ["/app/bin/lightning", "eval", "Lightning.Release.migrate"]

You shouldn't have to pass in a whole bunch of worker secrets and keys to this container only to run the migrations.

It would be great to be able to run migrations with the bare minimum in terms of secrets.

• db url (yes.)
• secret key base (maybe?)
• primary encryption key (maybe?)