build: stop make fetch_binaries downloading the beta site's index files

The beta site now has index.html and index.json directory listings
made by rclone index. fetch_binaries copied them into build/ where
make upload and make upload_github would have published them with the
release.

Makefile

@@ -285,7 +285,7 @@ endif
 
 # Fetch the binary builds from GitHub actions
 fetch_binaries:
-	rclone -P sync --exclude "/testbuilds/**" --delete-excluded $(BETA_UPLOAD) build/
+	rclone -P sync --exclude "/testbuilds/**" --exclude "/index.{html,json}" --delete-excluded $(BETA_UPLOAD) build/
 
 serve:	website
 	cd docs && hugo server --logLevel info -w --disableFastRender --ignoreCache