CI: don't build kernel each time

[lumag]

Rebuilding the kernel takes a lot of time (and compute power) during PR tests, however it's totally unnecessary: the goal is not to test the kernel, but to test the userspace app. The kernel should be compiled either on a scheduled bases or by manually triggering the pipeline at an important points after the release. FastRPC testing should use the prebuilt kernel images stored inside GitHub artifacts

[github-actions]

This issue has been marked as stale due to 60 days of inactivity.