Fix reference to protoc binary in protobuf-generate.cmake

This should fix #12374, #12375, and #12450. The `protobuf_PROTOC_EXEC` variable
is not defined, and I think `protobuf::protoc` is what we should be using
instead.

PiperOrigin-RevId: 525591320

cmake/protobuf-generate.cmake

@@ -137,7 +137,7 @@ function(protobuf_generate)
 
     add_custom_command(
       OUTPUT ${_generated_srcs}
-      COMMAND ${protobuf_PROTOC_EXE}
+      COMMAND protobuf::protoc
       ARGS ${protobuf_generate_PROTOC_OPTIONS} --${protobuf_generate_LANGUAGE}_out ${_plugin_options}:${protobuf_generate_PROTOC_OUT_DIR} ${_plugin} ${_protobuf_include_path} ${_abs_file}
       DEPENDS ${_abs_file} ${protobuf_PROTOC_EXE} ${protobuf_generate_DEPENDENCIES}
       COMMENT ${_comment}