store: Add the `%daemon-socket-file' parameter.

* guix/store.scm (%daemon-socket-file): New variable.
  (open-connection): Use it as the default value for FILE.

guix/store.scm

@@ -31,7 +31,9 @@
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 ftw)
   #:use-module (ice-9 regex)
-  #:export (nix-server?
+  #:export (%daemon-socket-file
+
+            nix-server?
             nix-server-major-version
             nix-server-minor-version
             nix-server-socket
@@ -143,6 +145,12 @@
   (string-append (or (getenv "NIX_STATE_DIR") %state-directory)
                  "/daemon-socket/socket"))
 
+(define %daemon-socket-file
+  ;; File name of the socket the daemon listens too.
+  (make-parameter (or (getenv "GUIX_DAEMON_SOCKET")
+                      %default-socket-path)))
+
+
 
 ;; serialize.cc
 
@@ -365,7 +373,7 @@
   (message nix-protocol-error-message)
   (status  nix-protocol-error-status))
 
-(define* (open-connection #:optional (file %default-socket-path)
+(define* (open-connection #:optional (file (%daemon-socket-file))
                           #:key (reserve-space? #t))
   "Connect to the daemon over the Unix-domain socket at FILE.  When
 RESERVE-SPACE? is true, instruct it to reserve a little bit of extra