Vagrantfile: Check for OS before patching the lxc-config.

Followup of e68b45d089
Without this vagrant up will fail on Windows.
https://chat.zulip.org/#narrow/stream/development.20help/topic/
Vagrant.20up.20error

Vagrantfile

@@ -31,6 +31,7 @@ end
 # have the box (e.g. on first setup), Vagrant would download it but too
 # late for us to patch it like this; so we prompt them to explicitly add it
 # first and then rerun.
+if Vagrant::Util::Platform.linux?
   if ['up', 'provision'].include? ARGV[0]
     LXC_VERSION = `lxc-ls --version`.strip unless defined? LXC_VERSION
     if LXC_VERSION == "2.1.0"
@@ -53,6 +54,7 @@ if ['up', 'provision'].include? ARGV[0]
       end
     end
   end
+end
 
 # Workaround: Vagrant removed the atlas.hashicorp.com to
 # vagrantcloud.com redirect in February 2018. The value of