services: nix: Export nix-configuration.

* gnu/services/nix.scm (nix-configuration, nix-configuration?): Export.

Signed-off-by: Oleg Pykhalov <go.wigust@gmail.com>

gnu/services/nix.scm

@@ -35,7 +35,10 @@
   #:use-module (ice-9 match)
   #:use-module (ice-9 format)
   #:use-module (guix modules)
-  #:export (nix-service-type))
+  #:export (nix-service-type
+
+            nix-configuration
+            nix-configuration?))
 
 ;;; Commentary:
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