[5.2] Fix issue #12628 Scheduler no longer runs commands in background

[jn-jairo]

Fix issue #12628

The fix adds a option runInBackground, so for long tasks just need add runInBackground() like the example:

$schedule->command("email:customers")->runInBackground()->withoutOverlapping();
$schedule->command("email:organisations")->runInBackground()->withoutOverlapping();
$schedule->command("email:administrators")->runInBackground()->withoutOverlapping();

[taylorotwell]

How exactly is this running them in the background?

[GrahamCampbell]

@taylorotwell See 9147bf7. In particular, read the full commit message. This PR reverts that change.

[taylorotwell]

But using Symfony process doesn't run commands in the background necessarily.

[taylorotwell]

Looks like this method doesn't trim off the &