Nix Flake for building a rust project that has a build.rs that references a file that is "../another_directory/file"

flake.nix

# https://github.com/ipetkov/crane/blob/master/examples/build-std/flake.nix
{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    crane = {
      url = "github:ipetkov/crane";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    flake-utils.url = "github:numtide/flake-utils";
    rust-overlay = {
      url = "github:oxalica/rust-overlay";
      inputs = {
        nixpkgs.follows = "nixpkgs";
        flake-utils.follows = "flake-utils";
      };
    };
  };
  outputs = { self, nixpkgs, crane, flake-utils, rust-overlay }:
    # let
    #   optionalList = cond: list: if cond then list else [ ];
    # in
    flake-utils.lib.eachDefaultSystem
      (system:
        let
          overlays = [ (import rust-overlay) ];
          pkgs = import nixpkgs {
            inherit system overlays;
          };
          rustToolchain = pkgs.pkgsBuildHost.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
          craneLib = (crane.mkLib pkgs).overrideToolchain rustToolchain;
          filterProto = path: _type: builtins.match ".*proto$" path != null;
          protoOrCargo = path: type:
            (filterProto path type)
            || (craneLib.filterCargoSources path type);
          # src = craneLib.cleanCargoSource (craneLib.path ./.);


          ##### PROBLEM: this flake lives in the "rust" directory.
          ##### PROBLEM: this does not find the file "../spec/service.proto", which is needed by ./build.rs
          ##### PROBLEM: 
          src = pkgs.lib.debug.traceVal (nixpkgs.lib.cleanSourceWith {
            src = craneLib.path ./.; # The original, unfiltered source
            filter = protoOrCargo;
          });

          my-crate = craneLib.buildPackage {
            inherit src;

            cargoVendorDir = craneLib.vendorMultipleCargoDeps {
              inherit (craneLib.findCargoFiles src) cargoConfigs;
              cargoLockList = [
                ./Cargo.lock

                # Unfortunately this approach requires IFD (import-from-derivation)
                # otherwise Nix will refuse to read the Cargo.lock from our toolchain
                # (unless we build with `--impure`).
                #
                # Another way around this is to manually copy the rustlib `Cargo.lock`
                # to the repo and import it with `./path/to/rustlib/Cargo.lock` which
                # will avoid IFD entirely but will require manually keeping the file
                # up to date!
                "${rustToolchain.passthru.availableComponents.rust-src}/lib/rustlib/src/rust/Cargo.lock"

                ###### BY THE WAY: I'm not sure what this comment above means or how to remove the IFD. I already keep the Cargo.lock up to date, so... ?
              ];
            };

            #### I don't remember why this is here...
            # cargoExtraArgs = "-Z build-std --target x86_64-unknown-linux-gnu";

            buildInputs = with pkgs; [
              protobuf
              rustToolchain
              gcc11  ###### For some reason, gcc11 does not seem to get used, and instead gcc12 is used...
              # protobufc
              # protoc-gen-rust
              # Add additional build inputs here
            ];
          };

          nativeBuildInputs = with pkgs; [
            rustToolchain
            gcc11  ###### For some reason, gcc11 does not seem to get used, and instead gcc12 is used...
            # pkg-config
            # protobuf
            # protobufc
            # protoc-gen-rust
            # protoc-gen-go
            # protoc-gen-go-grpc
          ];

          buildInputs = with pkgs; [
            gcc11  ###### For some reason, gcc11 does not seem to get used, and instead gcc12 is used...
            # protobuf
            # protobufc
            # protoc-gen-rust
            # openssl
          ];
        in
        {
          checks = {
            inherit my-crate;
          };

          packages.default = my-crate;

          devShells.default = pkgs.mkShell {
            inherit buildInputs nativeBuildInputs;
            inputsFrom = builtins.attrValues self.checks.${system};
            PROTOC = "${pkgs.protobuf}/bin/protoc";
          };
        }
      );
}