You have to think, what type of person would actually volunteer to serve on a firing squad? Yech.
I'm sure it's a paid position. perhaps even a union job ("correctional officer" and executioner are synonyms, we all know that). though you could probably get a few John Birch types to volunteer for such a job.
Well like all executions it's not frequent enough that "execution" through any means can be a full time job. So it's probably some other role one can take if they work in the prison system, and you can imagine the lawsuits and controversies that'd arise if a guard was ever told "execute this person or be fired". So I'd imagine all employees who serve on such a role are volunteers.