shadowfox: Add ldflags

shadowfox-updater compares the version of itself with that of the latest
version released upon startup. If it doesn't know it's own version, this
comparison always falsely leads to it suggesting the user to update it
upon startup, where in fact it is updated. This is a confusing behavior
only NixOS users suffer and now it's solved.

pkgs/tools/networking/shadowfox/default.nix

@@ -15,7 +15,9 @@ buildGoModule rec {
 
   doCheck = false;
 
-  buildFlags = [ "--tags" "release" ];
+  buildFlagsArray = [
+    "-ldflags=-X main.tag=v${version}"
+  ];
 
   meta = with stdenv.lib; {
     description = ''