check-available-binaries: Use 'with-store'.

* build-aux/check-available-binaries.scm: Use 'with-store' instead of an
  explicit 'open-connection'.

build-aux/check-available-binaries.scm

@@ -28,8 +28,8 @@
              (srfi srfi-1)
              (srfi srfi-26))
 
-(let* ((store  (open-connection))
-       (native (append-map (lambda (system)
+(with-store store
+  (let* ((native (append-map (lambda (system)
                                (map (cut package-derivation store <> system)
                                     (list %bootstrap-tarballs emacs)))
                              %supported-systems))
@@ -52,4 +52,4 @@
       (when result
         (format (current-error-port) "~a packages found substitutable~%"
                 (length total)))
-    (exit result)))
+      (exit result))))