Draft: Allow gRPC fork support to be used on Linux only

What does this MR do and why?

This prevents failures when GRPC.prefork is called on other systems.

References

• !205580 (comment 2810822916)

How to set up and validate locally

1. FILTER=09_issues bundle exec rake db:seed_fu
2. yes | gdk reset-data
3. Observe no errors on Mac and Linux

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.