fix: allow expandable env vars to reference any non-expandable env vars

[sapphi-red]

Description

This PR makes Vite to allow expandable env vars (the env vars that will be expanded by dotenv-expand) to reference any non-expandable env vars.

The downside and upside is the opposite of #18981.

Even with this PR,

# .env
VITE_APP_BASE_ROUTE=/
VITE_APP_BASE_URL=$VITE_APP_BASE_ROUTE
# .env.production
VITE_APP_BASE_ROUTE=/app/

still means

VITE_APP_BASE_URL=/app/
VITE_APP_BASE_ROUTE=/app/

fixes #18975
alternative to #18981

[sapphi-red]

I'm leaning towards this approach because it's easier to compose multiple env files, even if it means that our env files will work differently than others.