PR template: go back to old option name for now

The old one works on all nix(os) versions, whereas the new one doesn't
work e.g. on default 16.03 setting. Discussed on #15112.

Changed files
+2 -2
.github
+2 -2
.github/PULL_REQUEST_TEMPLATE.md
···
###### Things done
- [ ] 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
···
###### Things done
- [ ] Tested using sandboxing
+
([nix.useChroot](http://nixos.org/nixos/manual/options.html#opt-nix.useChroot) on NixOS,
+
or option `build-use-chroot` in [`nix.conf`](http://nixos.org/nix/manual/#sec-conf-file)
on non-NixOS)
- Built on platform(s)
- [ ] NixOS