Fix `x test --doc --stage 0 library/std`

I managed to break this in https://github.com/rust-lang/rust/pull/95449.
I am not quite sure why this is the correct fix, but it doesn't break `doc --stage 0`
and is strictly closer to the previous behavior.

Previously, rustdoc would error with strange issues because of the mismatched sysroot:
```
error[E0460]: found possibly newer version of crate `std` which `rustc_span` depends on
  --> /home/jnelson/rust-lang/rust/compiler/rustc_lint_defs/src/lib.rs:14:5
   |
14 | use rustc_span::{sym, symbol::Ident, Span, Symbol};
   |     ^^^^^^^^^^
   |
   = note: perhaps that crate needs to be recompiled?
   = note: the following crate versions were found:
           crate `std`: /home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-ff9290e971253a38.rlib
           crate `std`: /home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-ff9290e971253a38.so
           crate `rustc_span`: /home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release/deps/librustc_span-ed11dce30c1766f9.rlib
```

0 files changed, 0 insertions, 0 deletions