Revert "Revert "Drop tests.installer.grub1 as release critical""

This reverts commit cd28c5517c224c0c969f6bccb1f85dcc280ed7f2. Still broken:

http://hydra.nixos.org/build/23928152

nixos/release-combined.nix

···
         (all nixos.tests.firefox)
         (all nixos.tests.firewall)
         (all nixos.tests.gnome3)
-        (all nixos.tests.installer.grub1)
         (all nixos.tests.installer.lvm)
         (all nixos.tests.installer.luksroot)
         (all nixos.tests.installer.separateBoot)