`x clean compiler/rustc_middle` errors with a stability error

[Noratrieb]

On current master 4c8862b, I ran x clean compiler/rustc_middle in the root.

This doesn't clean anything but errors with

error: failed to load manifest for workspace member `/home/nils/projects/rust/library/std`

Caused by:
  failed to parse manifest at `/home/nils/projects/rust/library/std/Cargo.toml`

Caused by:
  the cargo feature `public-dependency` requires a nightly version of Cargo, but this is the `beta` channel
  See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information about Rust release channels.
  See https://doc.rust-lang.org/beta/cargo/reference/unstable.html#public-dependency for more information about using this feature.

looks like a RUSTC_BOOTSTRAP=1 is missing, setting the env var manually makes it work.