build: bump version to v2.6.4-beta (pre-release)

Author: reyyuuki

pubspec.yaml

@@ -1,7 +1,7 @@
 name: azyx
 description: "A new Flutter project."
 publish_to: "none"
-version: 2.6.4-beta+3
+version: 2.6.4-beta+4
 
 environment:
   sdk: ^3.5.0