Environment variable for vcpkg location should be VCPKG_ROOT

[michaelbprice]

The vcpkg community, including the maintainers, have coalesced around using the environment variable VCPKG_ROOT to designate the location of the tool. The images for the GitHub0-hosted runners should migrate away from using VCPKG_INSTALLATION_ROOT to VCPKG_ROOT as doing so will simplify the use of vcpkg across varying tools and environments.

[miketimofeev]

Hey, Michael!
Thanks for the heads up! I think it makes sense to keep both variables for some time to preserve backward compatibility. Do you have any objections?

[michaelbprice]

Yes, absolutely should keep both variables, with a clear indication in the documentation that VCPKG_INSTALLATION_ROOT is deprecated, and if possible make it clear when it will go away.