Auto merge of #58238 - Mark-Simulacrum:doctest-fix, r=alexcrichton

Fixes rustdoc in stage 0, stage 1

When a request for rustdoc is passed for stage 0, x.py build --stage 0
src/tools/rustdoc or ensure(tool::Rustdoc { .. }) with top_stage = 0, we
return the rustdoc for that compiler (i.e., the beta rustdoc).

This fixes stage 0 of https://github.com/rust-lang/rust/issues/52186 as well as being part of general workflow improvements (making stage 0 testing for std work) for rustbuild.

The stage 1 fix (second commit) completely resolves the problem, so this fixes https://github.com/rust-lang/rust/issues/52186.

diff --cc src/bootstrap/builder.rs
Simple merge

diff --cc src/bootstrap/tool.rs
Simple merge