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

This reverts commit 1907ca8a2aaa51d3e6910bb086dd75efffa81a6b. See

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

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)