github PR template: reformulate the sandboxing help

It's been reported that when running through nix daemon,
sandboxing can't be turned on by passing `--option` (silently ignored),
so let's not suggest that way.
https://github.com/NixOS/nixpkgs/issues/14866#issuecomment-214619909

.github/PULL_REQUEST_TEMPLATE.md

···
 ###### Things done
 
-- [ ] Tested using sandboxing (`nix-build --option build-use-sandbox true` or [nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS)
+- [ ] Tested using sandboxing
+  ([nix.useSandbox](http://nixos.org/nixos/manual/options.html#opt-nix.useSandbox) on NixOS,
+    or option `build-use-sandbox` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
+    on non-NixOS)
 - Built on platform(s)
    - [ ] NixOS
    - [ ] OS X