runtime: Environment.OSVersion returns wrong info on Big Sur

This returns 10.16 and may be caveat of the beta. However, is is curious that OS tools report 11.0 as expected:

furt@macos-11 foo % sysctl kern.osproductversion
kern.osproductversion: 11.0

furt@macos-11 foo % sw_vers
ProductName:	macOS
ProductVersion:	11.0
BuildVersion:	20A5343i

This seems to also impact dotnet --info and perhaps package selection.

cc: @akoeplinger @marek-safar @danmosemsft