Update msgpack org reference after transfer

The msgpack repo was transferred from seantallen-org to ponylang.

Author: SeanTAllen

.github/workflows/rebuild-ponyc-based-images.yml

@@ -715,7 +715,7 @@ jobs:
           - ponylang/uri
           - ponylang/valbytes
           - ponylang/web_link
-          - seantallen-org/msgpack
+          - ponylang/msgpack
     steps:
       - name: Send
         # v2.1.1