ci: add job to cleanup the unused local sstate-cache artifacts

[quaresmajose]

We need to clean up the ci local sstate-cache that is not used, otherwise it will grow infinitely.
This will take up significant space that will not be used by anyone since invalid sstate-cache artefacts will no longer be needed.

This can be done by adding a new job that runs after all builds are successful and we can do this using the upstream script in oe-core or call the rsync directly.

[doanac]

Maybe we do it as part of the nightly run?

[ricardosalveti]

Yeah, nightly is probably a good place to hook this up.