Run each powershell script in new powershell instance

Current windows provisioning procedure accumulates the environment from the previous provisioning steps. This leads to quite unpredictable results in case if packages litter the environment with their internals. One of the critical cases is the use of MSVC dev shell. If vcvars is called once it cannot be called second time with the different MSVC version. It could be that Qt dependencies need to be build with the specific MSVC versions and we should allow provisioning packages to specify this. The only solution I see by now is running new powershell instance for each provisioning script.