Reproducibility check: You need an x86_64 linux box, justbuild 1.2.0 or heigher, and some C compiler.
Check out https://github.com/just-buildsystem/bootstrappable-toolchain at commit 060a0cf338d6024eee37cc344c224fe3bcb78e81 (current head) and built the default target of the busybox-latest logical repository.
Can you confirm that only source archives are fetched and you obtain a directory with git tree identifier dafa90df3a392b283eb03092960b6166acf391ec that contains a statically-linked busybox?
GitHub - just-buildsystem/bootstrappable-toolchain: Collection of toolchains built from source

Collection of toolchains built from source. Contribute to just-buildsystem/bootstrappable-toolchain development by creating an account on GitHub.

GitHub
On nix, some paths have to be set, but it works as expected.