It is not possible to tie a matrix build to a node/label. The matrix build will
'roam' across all slaves.
If the number of executors per slave is set to 1, then the job may not finish if
the one of jobs it starts has to run on the machine the matrix job 'roams' to.
Setting this as a P1 as no work around - makes matrix builds useless on systems
with slaves with an executor of 1, there is no work around.