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

authored by Tuomas Tynkkynen and committed by Domen Kožar affcf922 0488cb18

+1
+1
nixos/release-combined.nix
··· 70 70 (all nixos.tests.installer.simple) 71 71 (all nixos.tests.installer.simpleLabels) 72 72 (all nixos.tests.installer.simpleProvided) 73 + (all nixos.tests.installer.simpleUefiSystemdBoot) 73 74 (all nixos.tests.installer.swraid) 74 75 (all nixos.tests.installer.btrfsSimple) 75 76 (all nixos.tests.installer.btrfsSubvols)