ci: switch appveyor to gce builders to try debugging the 259 failure
ci: switch appveyor to gce builders to try debugging the 259 failure

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