Make sure deploy to kubernets works only on main

What does this MR do?

Fixes the trigger to deploy to kubernetes to work only on main in the canonical repo

Why was this MR needed?

It's currently not being triggered on main

What's the best way to test this MR?

• Running a pipeline for the branch with the following variables simulating a commit in main works and has the deploy to k8s job included https://gitlab.com/gitlab-org/gitlab-runner/-/pipelines/1667626306:

• The pipeline in this MR does not include the deploy job

What are the relevant issue numbers?

Followup from Deploy each commit from main to kubernetes cluster (!5314 - merged) • Georgi N. Georgiev | GitLab • 17.9