CI: Switch to new Windows runners
gitlab will drop cmd.exe support with GitLab 13 so I took the opportunity to add new runners with Windows 2016 and powershell as default.
These runners are tagged with win32-ps instead of win32. The old runners will be switched off in the coming weeks.
The main difference is that all commands and env expansions use powershell and Windows 2016 instead of 2012r2.