ceph-windows-pull-requests: constrain to windows label
Currently, the intersection of focal && libvirt labels returns some
smithi (nproc == 8) and mira (nproc == 4) nodes, which are inadequate
for the two VMs that the Windows job spawns. It turns we already have
a windows label, which, after dropping it from mira015, seems to be on
point (i.e. would filter those low CPU nodes out). Let's use it!