Bootstrap build fails: found crate `serde_derive` compiled by an incompatible version of rustc

I did a git pull, fixed the submodules, and now I am trying to run ./x.py check. However, that fails:

$ ./x.py check
Building bootstrap
   Compiling build_helper v0.1.0 (/home/r/src/rust/rustc.3/src/tools/build_helper)
error[E0514]: found crate `serde_derive` compiled by an incompatible version of rustc
 --> /home/r/src/rust/rustc.3/src/tools/build_helper/src/metrics.rs:1:5
  |
1 | use serde_derive::{Deserialize, Serialize};
  |     ^^^^^^^^^^^^
  |
  = note: the following crate versions were found:
          crate `serde_derive` compiled by rustc 1.79.0-beta.1 (6b544f5ff 2024-04-28): /home/r/src/rust/rustc.3/build/bootstrap/debug/deps/libserde_derive-970fa0dc5f1e06a7.so
  = help: please recompile that crate using this compiler (rustc 1.79.0-beta.6 (66eb3e404 2024-05-23)) (consider running `cargo clean` first)

[...]

This must b a fairly recent regression, I've never had to manually clean bootstrap before.

Cc @onur-ozkan