vagrant: fix openbsd shell

Vagrantfile

@@ -510,6 +510,7 @@ Vagrant.configure(2) do |config|
     b.vm.provider :virtualbox do |v|
       v.memory = 1024 + $wmem
     end
+    b.ssh.shell = "sh"
     b.vm.provision "packages openbsd", :type => :shell, :inline => packages_openbsd
     b.vm.provision "build env", :type => :shell, :privileged => false, :inline => build_sys_venv("openbsd64")
     b.vm.provision "install borg", :type => :shell, :privileged => false, :inline => install_borg(false)