https://github.com/cnr/cgroup-rts-threads is marked a broken on Nix - Propose an upstream fix

[morucci]

Propose a fix upstream to remove the need for that override in default.nix.

[TristanCacqueray]

I don't think cgroup-rts-threads need a change, it's just the test that can't be ran inside a nix build as it validate a cgroup usage that is not allowed, iirc. Perhaps upstream nixpkgs would accept a change to disable the tests for this package, but it's also fine to just do this downstream.