Hydra job building them: https://hydra.nixos.org/build/208909151
The bootstrap files can be reproduced on the commit 21ec906463, e.g. by:
cat $(nix-build pkgs/top-level/release.nix -QA stdenvBootstrapTools.aarch64-linux.dist)/nix-support/hydra-build-products
file tarball /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/bootstrap-tools.tar.xz
file busybox /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/busybox
and the hashes as well:
nix hash file /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/bootstrap-tools.tar.xz
sha256-aJvtsWeuQHbb14BGZ2EiOKzjQn46h3x3duuPEawG0eE=
nix hash path /nix/store/kdpbw0plmjqlafjnpbz31ja51m4bd2dk-stdenv-bootstrap-tools/on-server/busybox
sha256-0MuIeQlBUaeisqoFSu8y+8oB6K4ZG5Lhq8RcS9JqkFQ=
You can check this on any machine, as the builds are on cache.nixos.org
but also you can reproduce the hashes when rebuilt on aarch64-linux HW.