Fix command being returned as Array

What does this merge request do and why?

When gdk.experimental.ruby_services is set to true, the GDK::Services::RailsBackgroundJobs#command method returned an Array instead of a String, resulting in sv/rails-background-jobs/run containing an invalid exec command:

main

$ grep 'exec chpst' sv/rails-background-jobs/run
exec chpst -P ["support/exec-cd", "gitlab", "bin/background_jobs", "start_foreground", "--timeout", "10"]

vs.

This branch (ashmckenzie/fix-rails-background-jobs-service)

$ grep 'exec chpst' sv/rails-background-jobs/run
exec chpst -P support/exec-cd gitlab bin/background_jobs start_foreground --timeout 10

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.