Re-activate snapshot packages and run checkpatch linter [REVPI-3716]

This MR intents to port the snapshot builds functionality, which was part of our GitHub CI.

• Run checkpatch on each commit of a merge request
• Build snapshot packages with kernelbakery (same code as on GitHub) after MR was merged. This helps to have always working kernel packages for default branch
• Build snapshot packages with kernelbakery (same code as on GitHub) optionally in a MR (user needs to start the job manually, grey icon in the pipeline)

Checkpatch review

Example of a successful job: https://gitlab.com/revolutionpi/linux/-/jobs/6038616561

Example of a failed job with checkpatch output: https://gitlab.com/revolutionpi/linux/-/jobs/6038614830

How to trigger build jobs

• Click on the >> symbol in the pipeline view
• Choose build target (or targes) by pressing the > button