rustc cannot determine correct SYSROOT for setups with shared components from sysroots

[Smorci]

Describe the bug

wasm-pack cannot find the right rustc SYSROOT when using rust-overlay

Expected behavior

Correct SYSROOT is used

Additional context

The changes that cause this bug were attempted to be fixed but in some cases this doesn't seem to be enough. Here is a more detailed explanation why that could be happening.

Posting this here because this behaviour might get hardcoded into rustc

Notify maintainers

@cstrahan @globin @Havvy

Metadata

 { nix-info } nix-info -m
 - system: `"x86_64-linux"`
 - host os: `Linux 6.1.11, NixOS, 23.05 (Stoat), 23.05pre453856.d917136f550`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.13.2`
 - channels(marci): `"home-manager-21.05.tar.gz"`
 - channels(root): `"agenix, nixos"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`