Windows build script tweaks

[heathhenley]

Just some minor tweaks I ran into trying to get build running on my environment in Win 11.

• Check that 7-zip is actually there - I didn't have it installed I guess
• Use vswhere to find a Visual Studio install to use - the hardcoded version didn't work for me because I have vs professional. The docs suggest that it's available for 2017 and onward.
• Some of the env variables representing paths were relative, I made them absolute
• Add a note to the docs about adding the dll to the search path, just like you're already doing in https://github.com/OpenNavigationSurface/BAG/blob/master/python/conftest.py#L5 (I was confused for a while about why I could run the tests but not use bagPy)

[selimnairb]

Thanks @heathhenley! Just one minor change.