Cleanup provisioner 'ui' usage

Without this commit, "ui" access is somewhat indirect in the
`PEBuild::Provisioner::PEBootstrap` class.

This commit simply cleans up the access to "ui" resource for the
"machine" context in  PEBuild::Provisioner::PEBootstrap class. This
class is accessing the "ui" resource by `@machine.env.ui` while the
`Vagrant::Machine` class supports more direct access via
`Vagrant::Machine#ui`.