buildserver: ensure dirs to mount cache exist in guest VM instance

It seems that the 9p synced folder setup is not as flexible and easy as the
VirtualBox one, so we have to do more little things like this.

buildserver/Vagrantfile

@@ -52,6 +52,10 @@ Vagrant.configure("2") do |config|
     config.vm.synced_folder configfile["cachedir"], '/vagrant/cache',
       create: true, type: synced_folder_type
   end
+  # Make sure dir exists to mount to, since buildserver/ is
+  # automatically mounted as /vagrant in the guest VM. This is more
+  # necessary with 9p synced folders
+  Dir.mkdir('cache') unless File.exists?('cache')
 
   # cache .deb packages on the host via a mount trick
   if configfile.has_key? "aptcachedir"