Revert "nix_unstable: disable due to broken data (#1565)"

This reverts commit ceb86e4.

This resolves #1565 because NixOS/nixpkgs#480323 has been merged as a
workaround to provide pname and version for packages lacking their
explicit definition.

Author: AiyionPrime

repos.d/nixos.yaml

@@ -74,4 +74,4 @@
   packagelinks:
     - type: PACKAGE_RECIPE
       url: 'https://github.com/NixOS/nixpkgs/blob/nixos-unstable/{?posfile}#L{?posline}'
-  groups: [ all, have_testdata, nix ]
+  groups: [ all, production, have_testdata, nix ]