meltano.yml does not allow referencing arbitrary environment variables from .env

Based on my understanding of meltano.yml, I would expect this to work:

.env:

WAREHOUSE_URL=postgres://localhost:5432/meltano

meltano.yml:

plugins:
  loaders:
  - name: target-postgres
    pip_url: git+https://github.com/meltano/target-postgres.git
    config:
      url: $WAREHOUSE_URL

However, with this configuration, this is the result:

$ meltano config target-postgres
[2020-07-15 12:23:01,556] [84379|MainThread|root] [WARNING] Variable WAREHOUSE_URL is missing from the environment.
{"url": null, "user": "warehouse", "password": "warehouse", "host": "localhost", "port": 5502, "dbname": "warehouse", "schema": null}

My hunch is that load_dotenv isn't called before meltano.yml is parsed.

This is not a blocker for us, the problem goes away if PG_URL is used instead, or if export WAREHOUSE_URL is used before meltano config. Just wanted to bring it up because I think most would find the behavior surprising.

I didn't add the bug labels because I'm not sure if this is actually a bug or if this behavior isn't supported.