fix: run dotenv-expand for each file separately

[sapphi-red]

Description

This PR makes the env resolution to match with docker-compose and bash, which was the reason of the breaking change of dotenv-expand v12 (motdotla/dotenv-expand#134 (comment)).

I tested the behavior of docker-compose with the following compose file.

services:
  app:
    image: busybox
    env_file:
      - .env
      - .env.production

The upside of this approach is that we will be aligned with other ecosystems.
The downside of this approach is that composing multiple env files will be difficult. For example,

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

means

VITE_APP_BASE_URL=/
VITE_APP_BASE_ROUTE=/app/

which is not likely most people expects.

fixes #18975