Auto merge of #85959 - RalfJung:miri-stage-0, r=Mark-Simulacrum

fix testing Miri with --stage 0

Fixes https://github.com/rust-lang/rust/issues/78778 for Miri.

The issue remains open for error_index_generator, which has its own dylib logic:
https://github.com/rust-lang/rust/blob/903e369c831d52726a5292f847e384622189d9a0/src/bootstrap/tool.rs#L396-L400
`@jyn514` I could just copy the logic from `add_rustc_lib_path`, but that does not seem great. Any other suggestions?

Also I wonder if maybe `prepare_tool_cargo` should already call `add_rustc_lib_path`.