Cannot use unstable flags in RUSTFLAGS_BOOTSTRAP

I want to compile rustc with some unstable flags.

RUSTFLAGS_BOOTSTRAP="-Zmeta-stats" x build compiler errors with

error: failed to run `rustc` to learn about target-specific information

Caused by:
  process didn't exit successfully: `/home/gh-Nilstrieb/rust/build/aarch64-unknown-linux-gnu/stage0/bin/rustc - --crate-name ___ --print=file-names -Wrust_2018_idioms -Wunused_lifetimes -Zmeta-stats --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 1)
  --- stderr
  error: the option `Z` is only accepted on the nightly compiler

  help: consider switching to a nightly toolchain: `rustup default nightly`

  note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

  note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>

  error: 1 nightly option were parsed

explicitly setting RUSTC_BOOTSTRAP=1 doesn't work either. using RUSTFLAGS= directly also doesn't work

Start by reproducing the reported RUSTFLAGS_BOOTSTRAP="-Zmeta-stats" x build compiler command and compare it with the behavior of RUSTC_BOOTSTRAP=1 and direct RUSTFLAGS. Trace how the bootstrap compiler is invoked and determine the expected handling of unstable flags before defining a targeted fix and regression test.