Use non-deprecated nixpkgs API

[K900]

See NixOS/nixpkgs#339072

CC @alyssais

[peterwaller-arm]

I just bisected an eval failure to this:

fenix/default.nix

Line 28 in 96a04a2

v = pkgs.stdenv.buildPlatform.rust.rustcTarget;

       error: attribute 'rust' missing
       at /nix/store/by17ag6v7rp41pw1qvq3ync4c5z5bync-source/default.nix:28:7:
           27|
           28|   v = pkgs.stdenv.buildPlatform.rust.rustcTarget;
             |       ^
           29|
       Did you mean rustc?

Repro command: NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix eval --impure --raw --override-input fenix . github:elfshaker/elfshaker.

[peterwaller-arm]

Ah, just realised it was using the 23.05 nixpkgs release branch. Bumping nixpkgs to 24.05 fixes it, as expected.