Add malvex-nixos-x86_64 worker

[malvex]

Closes #657

[malvex]

moved to NO_TIER, should be good now

[StanFromIreland]

I forced a run, I see we have a few failures, test.test_os.test_os, test_site and test_subprocess. Please open issues (if they do not have any already) for them.

[malvex]

I checked the errors, and it looks like it's caused by FHS env (basically sandbox), which prevents setuid/setgid operations.
I will take the runner temporarily offline and I will check the failed tests if any of them are actual bugs, and after I will rebuild the runner without sandbox (I assume it's unwanted for buildbot)