packaging: windows: optionally skip windeployqt

This is a big time saver in the context of CI testing where we only need to supply some env vars instead.

It would also be nice to also have the env vars filled out for CMake-generated project runtime configs to prevent unnecessary deployment for development builds, but that's not done in this commit.

Change-Id: I338827cb87c9fc71c9a6b4b3076b22aa7cf7cfa3