Allow passing environment variables to scheduled tasks

[jradtilbrook]

With this change, it allows us to pass environment variables to scheduled tasks similar to the Process facade.

Useful for commands or any other schedule tasks that need to override the environment variable for some uses but not all.

Example routes/console.php:

<?php

// process podcasts in default english
Schedule::command('app:process-podcasts')->everyHour();
// process podcasts in french
Schedule::command('app:process-podcasts')->env(['LANG' => 'fr'])->everyHour();

// prior:
Schedule::exec('LANG=fr php artisan app:process-podcasts')->everyHour();

This is only applicable to ::exec() and ::command() tasks that use the Symfony process package. ::job() and ::call() do not use it. As such, the method is overridden to throw an exception similar to runInBackground()