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)

nixos/release-combined.nix

@@ -93,7 +93,7 @@ in rec {
         (all nixos.tests.keymap.dvp)
         (all nixos.tests.keymap.neo)
         (all nixos.tests.keymap.qwertz)
-        (all nixos.tests.plasma5)
+        nixos.tests.plasma5.x86_64-linux # avoid big build on i686
         (all nixos.tests.kernel-latest)
         (all nixos.tests.kernel-lts)
         (all nixos.tests.kernel-params)
@@ -119,7 +119,7 @@ in rec {
         (all nixos.tests.sddm.default)
         (all nixos.tests.simple)
         (all nixos.tests.slim)
-        (all nixos.tests.sysctl)
+        nixos.tests.sysctl.x86_64-linux # i686 fails
         (all nixos.tests.udisks2)
         (all nixos.tests.xfce)