nixos tests: restrict sysctl and plasma5 to x86_64-linux
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)

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