Make modify_execution_info additive

We were already using it like it was, but apparently it's not additive
without the --incompatible_modify_execution_info_additive flag set.

Author: jjudd

.bazelrc_shared

@@ -15,6 +15,7 @@ build --verbose_failures
 build --worker_max_instances=4
 build --worker_sandboxing
 
+common --incompatible_modify_execution_info_additive
 common --experimental_output_paths=strip
 common --modify_execution_info=CppArchive=+supports-path-mapping
 common --modify_execution_info=CppCompile=+supports-path-mapping