Run mklink through cmd so windows postinstall works (#7982)

npm install fails on Windows in postinstall with "mklink: command not
found". mklink is a cmd.exe builtin rather than an executable, so the sh
running this script cannot exec it, and every symlink the install needs is
made this way.

The doubled slashes are the usual MSYS escaping, and are safe here because
this branch only runs when the OS was detected as cygwin or mingw.

ci/build/npm-postinstall.sh

@@ -18,7 +18,7 @@ symlink() {
   dest="$2"
   rm -rf "$dest"
   case $OS in
-    windows) mklink /J "$dest" "$source" ;;
+    windows) cmd //c mklink //J "$dest" "$source" ;;
     *) ln -s "$source" "$dest" ;;
   esac
 }