build(flake/outputs/foundry): Use rustPlatform from nixos-24.05

In the `nixos-unstable` nixpkgs branch, `rustPlatform` was updated to 1.80,
which is incompatible [1] with Foundry. Switch `rustPlatform` to the instance
from `nixos-24.05`, which is 1.77.0x [1].

[1]: https://github.com/metacraft-labs/nixos-modules/actions/runs/10542536080/job/29209445205?pr=150
[2]: https://github.com/NixOS/nixpkgs/blob/797f7dc49e0bc7fab4b57c021cdf68f595e47841/pkgs/top-level/all-packages.nix#L16497

packages/default.nix

@@ -79,7 +79,10 @@
         inherit (pkgs) terraform;
         inherit (legacyPackages.inputs.terranix) terranix;
         inherit (legacyPackages.inputs.dlang-nix) dcd dscanner serve-d dmd ldc;
-        inherit (legacyPackages.inputs.ethereum-nix) mev-boost nethermind web3signer foundry nimbus-eth2;
+        inherit (legacyPackages.inputs.ethereum-nix) mev-boost nethermind web3signer nimbus-eth2;
+        foundry = legacyPackages.inputs.ethereum-nix.foundry.override {
+          rustPlatform = pkgs.rustPlatform;
+        };
       };
   };
 }