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

This reverts commit 1907ca8a2aaa51d3e6910bb086dd75efffa81a6b. See

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

+1
+1
nixos/release-combined.nix
··· 52 (all nixos.tests.firefox) 53 (all nixos.tests.firewall) 54 (all nixos.tests.gnome3) 55 (all nixos.tests.installer.lvm) 56 (all nixos.tests.installer.luksroot) 57 (all nixos.tests.installer.separateBoot)
··· 52 (all nixos.tests.firefox) 53 (all nixos.tests.firewall) 54 (all nixos.tests.gnome3) 55 + (all nixos.tests.installer.grub1) 56 (all nixos.tests.installer.lvm) 57 (all nixos.tests.installer.luksroot) 58 (all nixos.tests.installer.separateBoot)