Setup cross-triggering across CI workflows

[umarcor]

Currently, all workflows are triggered at the same time.
That produces some races and some tools are built twice in the same run (moby/buildkit#1930).
Workflows are "dispatchable" and we do know how to trigger them (.github/trigger.sh).
We didn't implement it yet because we'd like to be automatically sychronised with the Graphs.
As explained in Graph generation/parsing, script map.py is work-in-progress to extract a hierarchy of images/jobs from the config.yml file and from the workflows.