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)

[encukou]

Feel free to keep it running; unstable ones are expected to have issues.
Once it's green for a while, mark it stable (despite the NixOS channel name). That's when people other than you will start paying attention to failures.

[vstinner]

If it's possible to detect that chown(), setregid() or similar function fail whereas they should not (detect the sandbox), maybe we can decorate the impacted tests to skip them. Disabling the sandbox is another easy option.

[malvex]

Thanks for clarification! The builder is back up and running.

I think it probably brings more value to run these tests rather than skipping them just because of sandbox, so disabled it (took a few tries to set up env variables correctly to tell it where the libraries are located), and now all the tests pass when running make directly.
Meanwhile builder got some jobs that failed somewhere else, so I will see what's that about and what can be done, but it already looks better