APP_KEY cant be set with key:generate when APP_KEY is already set in the environment

[nckrtl]

Consider this PR a bug report. I'm unsure yet what an elegant fix would be or if its intended behavior.

When running php artisan key:generate while APP_KEY has already been set in the environment the APP_KEY won't be set and a misleading error message "Unable to set application key. No APP_KEY variable was found in the .env file." will be shown.

You can also reproduce this on a mac or linux machine:

1. Clone and set up a new vanilla Laravel application.
2. Run export APP_KEY=abc on the command line.
3. Run php artisan key:generate inside the application.
4. Message "Unable to set application key. No APP_KEY variable was found in the .env file." will be shown.
5. Clear the environment APP_KEY with unset APP_KEY on the command line, and the key:generate command will succeed.

This issue also occurs when generating Laravel apps through jobs with Horizon as the APP_KEY will be set in the environment when jobs are being processed.

If this is expected behavior then at least a proper error message should be shown. Especially for LLMs this would be helpful.

I provided a couple of tests where test_key_generate_updates_env_file_even_when_app_key_exists_in_environment demonstrates the the current behavior and fails.

[taylorotwell]

PR to correct error message welcome!