appveyor: Use VS2017 for all our images
appveyor: Use VS2017 for all our images

This was [recommended by AppVeyor][1] to see if it has any impact on our
build times, hopefully on the beneficial side of things! This shouldn't
affect our binary compatibility for generated compilers like it would
normally do for Linux.

[1]: https://help.appveyor.com/discussions/questions/29832-did-recent-changes-apply-to-possibly-slow-down-builds#comment_46484879

graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust