fix(microfrontends): ensure that local proxy is loose (#10289)
### Description
The MFE proxy is a persistent task that isn't cached. We don't need to
get in user's way when they're trying to pass in an environment variable
to be used by the local proxy.
### Testing Instructions
Test that a `proxy` script gets an env var.
- Setup a custom proxy script in `package.json`: `echo Starting with
$PROXY_VAR && cat`
- Run `PROXY_VAR=foo turbo_dev --skip-infer dev -F nextjs-app-marketing`
- Verify that `Starting with foo` is in the proxy logs