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

This reverts commit 1907ca8a2aaa51d3e6910bb086dd75efffa81a6b. See

https://github.com/NixOS/nixpkgs/commit/1907ca8a2aaa51d3e6910bb086dd75efffa81a6b#commitcomment-9719007

Changed files
+1
nixos
+1
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)