Update install-mingw.sh

Author: ChrisDenton

src/ci/scripts/install-mingw.sh

@@ -46,5 +46,5 @@ if isWindows && isKnownToBeMingwBuild; then
 
     # Workaround for spurious ci failures
     # see https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Spurious.20bors.20CI.20failures/near/528915775
-    mkdir /c/msys64/home/runneradmin 
+    mkdir -p ~
 fi