rustdoc: use current stage if download-rustc enabled When using download-rustc, using stage 1 rustdoc results in the wrong librustc_driver being used. ```sh $ ./build/host/stage1/bin/rustdoc --version ./build/host/stage1/bin/rustdoc: error while loading shared libraries: librustc_driver-7ff02ed05016d515.so: cannot open shared object file: No such file or directory ``` This change fixes that by not cutting the stage if download-rustc is enabled.