Auto merge of #56201 - kennytm:revert-55935, r=alexcrichton
Auto merge of #56201 - kennytm:revert-55935, r=alexcrichton

Revert "appveyor: Use VS2017 for all our images"

This reverts commit 008e5dcbd55cd751717ffd35a51dd65cd40011d4 (#55935)

We suspect this causes the spurious failure in https://github.com/rust-lang/rust/pull/55906#issuecomment-441365922 and https://github.com/rust-lang/rust/pull/55915#issuecomment-441377543.

r? @alexcrichton

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