[YoutubeDL] Apply expand_path after output template substitution

os.path.expandvars translates '%%' into '%' making output template invalid.
Before: '%%(ext)s' -(expand path)-> '%(ext)s' -(outtmpl subst.)-> 'mp4'
After: '%%(ext)s' -(outtmpl subst.)-> '%(ext)s' -(expand path)-> '%(ext)s'

youtube_dl/YoutubeDL.py

@@ -672,8 +672,7 @@ class YoutubeDL(object):
                         FORMAT_RE.format(numeric_field),
                         r'%({0})s'.format(numeric_field), outtmpl)
 
-            tmpl = expand_path(outtmpl)
+            filename = expand_path(outtmpl % template_dict)
-            filename = tmpl % template_dict
             # Temporary fix for #4787
             # 'Treat' all problem characters by passing filename through preferredencoding
             # to workaround encoding issues with subprocess on python2 @ Windows