distro-build/installer-dmg: codesign whole ".app" instead of just exe

This change is needed for the more stringent Gatekeeper introduced in
Mac OS X 10.9.5, but the build machine has to be 10.9 or later to make
10.9.5 clients happy.

pkgs/distro-build-pkgs/distro-build-client/installer-dmg.rkt

@@ -86,7 +86,7 @@
                        (path-replace-suffix name #"")))
         (define exe (build-path f "Contents" "MacOS" name))
         (when (file-exists? exe)
-          (system*/show codesign "-s" sign-identity exe)))))
+          (system*/show codesign "-s" sign-identity f)))))
   (check-bins (build-path dest-dir "bin"))
   (check-bins (build-path dest-dir "lib"))
   (check-apps dest-dir)