Expand VS search paths for old toolchains installed by new VS

[msarahan]

VS 2019 has a way to install older toolchains. If you choose vs2017 tools (14.1) to be installed, they go into a folder that our activation scripts do not know to look in.

https://github.com/AnacondaRecipes/aggregate/blob/master/vs2017/meta.yaml

cc @isuruf