Add manual tag to //objectivec:gen_wkt_sources since it should only be run internally by regenerate_stale_files.sh

Fixes https://github.com/protocolbuffers/protobuf/issues/12609

PiperOrigin-RevId: 528535687

objectivec/BUILD.bazel

diff --git a/objectivec/BUILD.bazel b/objectivec/BUILD.bazel
index 0965de5..87f1ec0 100644
--- a/objectivec/BUILD.bazel
+++ b/objectivec/BUILD.bazel
@@ -45,6 +45,7 @@
         for ext in _OBJC_EXTS
     ]),
     exec_tools = ["//:protoc"],
+    tags = ["manual"],
 )
 
 staleness_test(