build: Fix default value of `%state-directory'.

* guix/config.scm.in (%state-directory): Append `/nix'.
  Reported by Andreas Enge <andreas@enge.fr>.

guix/config.scm.in

@@ -46,7 +46,8 @@ (define %store-directory
   "@storedir@")
 
 (define %state-directory
-  "@guix_localstatedir@")
+  ;; This must match `NIX_STATE_DIR' as defined in `daemon.am'.
+  "@guix_localstatedir@/nix")
 
 (define %system
   "@guix_system@")