Add CI support for upstream RT stable
## Issue description
In order to help with upstream maintenance, and in turn improve the quality of the code going into the CIP RT kernels, we want to start CI build/testing the RT project's code.
## Useful links
- [linux-rt-devel](https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/)
- [linux-stable-rt](https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-stable-rt.git/)
## Acceptance criteria
TODO
issue