Set $NIX_DEBUG_INFO_DIRS when environment.enableDebugInfo is enabled

This allows it to co-exist with other debug info directories, such as
the one used by dwarffs
(https://github.com/edolstra/dwarffs/blob/master/module.nix).

nixos/modules/config/debug-info.nix

@@ -30,14 +30,15 @@ with lib;
   };
 
 
-  config = {
+  config = mkIf config.environment.enableDebugInfo {
 
     # FIXME: currently disabled because /lib is already in
     # environment.pathsToLink, and we can't have both.
     #environment.pathsToLink = [ "/lib/debug/.build-id" ];
 
-    environment.extraOutputsToInstall =
-      optional config.environment.enableDebugInfo "debug";
+    environment.extraOutputsToInstall = [ "debug" ];
+
+    environment.variables.NIX_DEBUG_INFO_DIRS = [ "/run/current-system/sw/lib/debug" ];
 
   };