Fix webamp-modern build outputs in turbo.json

The webamp-modern#build task had outputs set to an empty array,
which meant turbo wouldn't cache/restore the build directory.
This caused the deploy script to fail when the mv command couldn't
find the build directory on cache hits.

Changed outputs from [] to ["build/**"] to properly cache and
restore the build output.

Author: captbaritone

turbo.json

@@ -48,7 +48,7 @@
       "outputs": []
     },
     "webamp-modern#build": {
-      "outputs": []
+      "outputs": ["build/**"]
     },
     "webamp-modern#test": {
       "outputs": []