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