Incompatible flag to stop merging the CC toolchain's all_files into rust toolchain's all_files

[dzbarsky]

the CC toolchain's all_files can be quite big when using a hermetic sysroot, which creates large input trees for actions using this depset, including cargo_build_script. Note that cargo_build_script also explicitly adds the cc toolchain's all_files as well, but #3679 provides a way to avoid that when the rule author knows the cc toolchain isn't required. Other actions should likely make their own decisions whether they want to pull in the CC sysroot or not, and if so, do it explicitly.