Introduce SUPPRESS_JOBS CI variable
Sometimes, a job isn't expected to pass (most often `softload_gnu`, because certain components don't support non-Norcroft toolchains). By defining this variable in a `.gitlab-ci.yml` file, we can force any of the jobs to be skipped. This only applies to submodule pipelines: it is not sensible to skip a build phase for a superproject pipeline.
suppress-jobs in 3 seconds (queued for 1 second)1 job for