Drop the EFI installer tests from the release-critical builds for now

They fail randomly due to a hard-coded 30-second timeout in udev
waiting for /dev/sda1 to appear:

  systemd-udevd[1151]: worker [1168] /devices/pci0000:00/0000:00:04.0/host2/target2:0:0/2:0:0:0/block/sda/sda1 timeout; kill it
  systemd-udevd[1151]: seq 1059 '/devices/pci0000:00/0000:00:04.0/host2/target2:0:0/2:0:0:0/block/sda/sda1' killed

Hopefully we can use virtio in the future for the EFI tests.

http://hydra.nixos.org/build/6695897