nixos/tests: Make the UEFI boot test release-critical

Seems to be working fine, based on the history:
https://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.installer.simpleUefiGummiboot.i686-linux/all
https://hydra.nixos.org/job/nixos/trunk-combined/nixos.tests.installer.simpleUefiGummiboot.x86_64-linux/all

Changed files
+1
nixos
+1
nixos/release-combined.nix
···
(all nixos.tests.installer.simple)
(all nixos.tests.installer.simpleLabels)
(all nixos.tests.installer.simpleProvided)
+
(all nixos.tests.installer.simpleUefiSystemdBoot)
(all nixos.tests.installer.swraid)
(all nixos.tests.installer.btrfsSimple)
(all nixos.tests.installer.btrfsSubvols)