nixos tests: restrict sysctl and plasma5 to x86_64-linux

- sysctl is new and never succeeded on i686-linux
> cannot stat /proc/sys/net/core/bpf_jit_enable: No such file or directory
- testing plasma5 on i686 would defeat part of the reason why we ended
supporting i686 (lots of stuff built on Hydra)

+2 -2
+2 -2
nixos/release-combined.nix
··· 93 93 (all nixos.tests.keymap.dvp) 94 94 (all nixos.tests.keymap.neo) 95 95 (all nixos.tests.keymap.qwertz) 96 - (all nixos.tests.plasma5) 96 + nixos.tests.plasma5.x86_64-linux # avoid big build on i686 97 97 (all nixos.tests.kernel-latest) 98 98 (all nixos.tests.kernel-lts) 99 99 (all nixos.tests.kernel-params) ··· 119 119 (all nixos.tests.sddm.default) 120 120 (all nixos.tests.simple) 121 121 (all nixos.tests.slim) 122 - (all nixos.tests.sysctl) 122 + nixos.tests.sysctl.x86_64-linux # i686 fails 123 123 (all nixos.tests.udisks2) 124 124 (all nixos.tests.xfce) 125 125