[vim] Fix escaping of fzf binary path containing spaces on Windows

Fix #2992

plugin/fzf.vim

@@ -464,7 +464,7 @@ try
   let temps  = { 'result': s:fzf_tempname() }
   let optstr = s:evaluate_opts(get(dict, 'options', ''))
   try
-    let fzf_exec = fzf#shellescape(fzf#exec())
+    let fzf_exec = (&shell =~ 'powershell' ? '&' : '') . shellescape(fzf#exec())
   catch
     throw v:exception
   endtry