Auto merge of #41996 - alexcrichton:more-retry, r=Mark-Simulacrum
Auto merge of #41996 - alexcrichton:more-retry, r=Mark-Simulacrum

appveyor: Wrap handle.exe download in a retry

Should help deal with spurious download failures.

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