Provide access to commit hash of a toolchain

[Nemo157]

I would like to setup remapping of rust toolchain's CI paths to the local rust-src component, something similar to

rust-toolchain = pkgs.rust-bin.selectLatestNightlyWith (toolchain:
  toolchain.default.override {
    extensions = ["rust-src"];
  }
);

"--remap-path-prefix=/rustc/${rust-toolchain.commit}=/${rust-toolchain}/lib/rustlib/src/rust"

But afaict there's no way to access the commit hash of a selected toolchain.