ensure missing -i still works

The deprecation warning is there, but it should keep working though!

src/cli.js

@@ -353,6 +353,7 @@ async function build() {
   // TODO: Deprecate this in future versions
   if (!input && args['_'][1]) {
     console.error('[deprecation] Running tailwindcss without -i, please provide an input file.')
+    input = args['--input'] = args['_'][1]
   }
 
   if (input && !fs.existsSync((input = path.resolve(input)))) {