[5.x] Fix runaway memory usage with horizon:listen

[cosmastech]

To resolve #1715

So it turns out that calling Symfony's Process with an array vs a string produces different behavior. Who knew? It seems that by passing a string, it calls sh <command> rather than exec <command> and the PID belongs to the sh process, not the actual command. Killing that process doesn't kill the underlying supervisor + workers.

Note that the identified problem doesn't appear to be an issue on bare metal on my MBP. However, inside of a docker container, big sadness.

[cosmastech]

@saifali-dev would you want to make this change locally and confirm it works for you?

[saifali-dev]

Thanks @cosmastech. I tested the PR locally (dev-pr-1716) in my app and can confirm it fixes the issue on my end. After multiple horizon:listen restarts, the Horizon master process no longer duplicates.