nixos/release: Replace a: b: a // b by mergeAttrs

No change in functionality, it just looks nicer that way.

Signed-off-by: aszlig <[e-mail]>

nixos/release.nix

···
   # If the test is only for a particular system, use only the specified
   # system instead of generating attributes for all available systems.
   in if args ? system then discover (import fn args)
-     else foldAttrs (a: b: a // b) {} (map discoverForSystem supportedSystems);
+     else foldAttrs mergeAttrs {} (map discoverForSystem supportedSystems);
 
   pkgs = import nixpkgs { system = "x86_64-linux"; };