[ci] Explicitly "free" 0-core binding of mpirun

Binding to core 0 is default behaviour of mpirun(1):

Please note that mpirun automatically binds processes as of the start of the v1.8 series. Three binding patterns are used in the absence of any further directives:

Bind to core: when the number of processes is <= 2
Bind to socket: when the number of processes is > 2
Bind to none: when oversubscribed