I've set Xvfb's option "Start only on nodes labeled" to "master" in a multi-configuration job.
None of the involved slave nodes has a Xvfb executable, only the master node has.
None of the combinations run on master, but I have a post-build script that needs Xvfb.
The job's "Restrict where this project can be run" is set to "master".
I would have expected Xvfb to run on the master during the whole job run, including the post-build phase, but it didn't happen.
tl;dr a matrix job spawning jobs on nodes without Xvfb, and a post-build jobs that collect such jobs results and creates plots from master.