Motivation for this change

Datadog complains about permissions to access tracefs every run:

dd-agent:2018-02-14 11:00:40,697 | WARNING | dd.collector | checks.disk(disk.py:106) | Unable to get disk metrics for /sys/kernel/debug/tracing: [Errno 13] Permission denied: '/sys/kernel/debug/tracing'
Being unable to imagine a scenario where this FS needs monitoring, I assume it's wise to exclude it.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.