Auto merge of #126728 - onur-ozkan:stage1-rustdoc, r=Kobzol

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.

Author: bors