[legacy-framework] Fix APP_ENV loading to not load all .env files

[JuanM04]

What are the changes and their implications?

Changes the behavior of blitz --env. Right now, if I run blitz start --env staging it will load both .env.production* and .env.staging*.

With these changes, only the env files declared with --env will be loaded. Also, now you can pass multiple environments:

blitz start
# .env.production.local
# .env.production
# .env.local
# .env

blitz start --env staging
# .env.staging.local
# .env.staging
# .env.local
# .env

blitz start --env staging,production
# .env.staging.local
# .env.staging
# .env.production.local
# .env.production
# .env.local
# .env

Feature Checklist

• Integration test added (see test docs if needed)
• Documentation added/updated (submit PR to blitzjs.com repo canary branch)

[flybayer]

Ah oops, thank you!

You do have one failing integration test, "test/integration/env-config-app-env/" under CI integration 7 https://github.com/blitz-js/blitz/runs/4422960457?check_suite_focus=true. The other failures are CI flake