Stop uninstalling/reinstalling the package each time on Windows.

[remeh]

As the MSI is creating a custom user, the behavior of uninstalling/reinstalling was creating a new user each time.

[olivielpeau]

@remeh note: Starting with 6.11.0, the agent MSI won't allow downgrades anymore. Without this change I think there would be no impact on the cookbook (since for a downgrade it would uninstall the Agent and then install the lower version), but with this change I think this would need to be handled differently... Could you give the downgrade a try as well? We can then figure out whether we need to do anything specific for downgrades (or just add docs, or do nothing if everything works)

[albertvaka]

I can confirm this change breaks the downgrades. We need a way to use the previous behaviour conditionally if we are downgrading.