store: Honor $NIX_STORE_DIR and $NIX_STATE_DIR.

* guix/store.scm (%nix-state-dir): Honor $NIX_STATE_DIR.
  (%store-prefix): Honor $NIX_STORE_DIR.

guix/store.scm

@@ -111,7 +111,8 @@
   (sha1 2)
   (sha256 3))
 
-(define %nix-state-dir "/nix/var/nix")
+(define %nix-state-dir
+  (or (getenv "NIX_STATE_DIR") "/nix/var/nix"))
 (define %default-socket-path
   (string-append %nix-state-dir "/daemon-socket/socket"))
 
@@ -437,7 +438,8 @@ file name.  Return #t on success."
 
 (define %store-prefix
   ;; Absolute path to the Nix store.
-  (make-parameter "/nix/store"))
+  (make-parameter (or (and=> (getenv "NIX_STORE_DIR") canonicalize-path)
+                      "/nix/store")))
 
 (define (store-path? path)
   "Return #t if PATH is a store path."