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 (all nixos.tests.installer.simple) 71 (all nixos.tests.installer.simpleLabels) 72 (all nixos.tests.installer.simpleProvided) 73 (all nixos.tests.installer.swraid) 74 (all nixos.tests.installer.btrfsSimple) 75 (all nixos.tests.installer.btrfsSubvols)
··· 70 (all nixos.tests.installer.simple) 71 (all nixos.tests.installer.simpleLabels) 72 (all nixos.tests.installer.simpleProvided) 73 + (all nixos.tests.installer.simpleUefiSystemdBoot) 74 (all nixos.tests.installer.swraid) 75 (all nixos.tests.installer.btrfsSimple) 76 (all nixos.tests.installer.btrfsSubvols)