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

@@ -28,6 +28,7 @@ common:tests --@rules_scala_annex//rules/scala:scala-toolchain=test_zinc_2_13
 
 # These are backwards incompatible options; we should check to see if their values have been flipped
 # when upgrading to new major Bazel version.
+common --incompatible_modify_execution_info_additive
 common --incompatible_auto_exec_groups
 common --incompatible_autoload_externally=sh_binary # sh_binary is used by rules_jvm_external
 common --incompatible_config_setting_private_default_visibility