feat: copy `.env` file in standalone mode (#34143)
Ref: #33897
I took a naive approach and simply added `.env` to the files that need to be copied.
Do we want to include `.env.production` as well? Ref: https://nextjs.org/docs/basic-features/environment-variables#loading-environment-variables
(I haven't tested what happens if the file does not exist on copy.)
Other files like `.env.*local` or `.env.{test,development}` don't make sense to copy.
## Bug
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
## Feature
- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`
## Documentation / Examples
- [ ] Make sure the linting passes by running `yarn lint`
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>
