fix: run mklink through cmd so windows install works

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.

Routing it through cmd works. Verified on Windows 11 with git-bash: `mklink
/J link src` reports command not found, `cmd //c mklink //J link src`
reports "Junction created" and the junction resolves. Tested with the same
relative paths symlink() passes, from a directory it has cd'd into.

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.

Author: denusklo

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
 }