Fix powershell stdin data race

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do?

Move encoder from global scope to method

Why was this MR needed?

Data race : https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/9656883916/viewer#L4252

What's the best way to test this MR?

What are the relevant issue numbers?