allow CI to skip builds of unrelated projects

[ggutierrez-sunbright]

with the recent addition of rdock-utils python package, the CI is doing unnecessary work compiling rDock when only rdock-utils has changes or viceversa.
in the future, there will be even more independent flows, for example, the ones that will build docker base or dev images.

so it is now obvious that we need to skip these pipelines, but the successful result on specific jobs is required as part of the main branch protection mechanism.
this doesn't allow us to use the general skip based on files changed, and we need to implement it in a different way, where, if there are no changes in the files/folders of interest, the job doesn't run steps and finish successfully immediately.

hopefully, there will be a 'simple', scalable and not very verbose way to do this