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

nixos/release-combined.nix

@@ -70,6 +70,7 @@ in rec {
         (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)