Add a Nix pkg to a GitHub CI Action

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Hello everyone!

I am trying to add a binary dependency in a GitHub CI using a flake.
The cargo test in some configs will depend on a binary located in BITCOIND_EXEC environment variable.
The codebase is extent, I don’t want to “nixify” the whole CI now. I just want to avoid download the external binary.

This is my flake.nix:

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    # pinned to 0.25.0
    nixpkgs-bitcoind.url = "github:nixos/nixpkgs?rev=9957cd48326fe8dbd52fdc50dd2502307f188b0d";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, nixpkgs-bitcoind, flake-utils, ... }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
        pkgs-bitcoind = nixpkgs-bitcoind.legacyPackages.${system};
      in
      {
        devShells.default = pkgs.mkShell
          {
            packages = [
              pkgs.bashInteractive
              pkgs-bitcoind.bitcoind
            ];
            shellHook = ''
              bitcoind --version
            '';
            BITCOIND_EXEC = "${pkgs.bitcoind}/bin/bitcoind";
          };
      });

Then how do I make sure that CI will have the packages installed and the BITCOIND_EXEC var defined?
Locally I am using nix-direnv that already does that to my local environment upon entering the directory.

This is the relevant parts in my GH Action:

  build-test:
    name: Build and test
    runs-on: ubuntu-latest
    strategy:
      matrix:
        rust:
          - version: stable
            clippy: true
          - version: 1.57.0 # MSRV
        features:
          - --no-default-features
          - --all-features
<....>

    steps:
      - name: checkout
        uses: actions/checkout@v2
      - name: Install Nix
        uses: DeterminateSystems/nix-installer-action@v5
      - name: Install Rust toolchain
        uses: actions-rs/toolchain@v1
        with:
            toolchain: ${{ matrix.rust.version }}
            override: true
            profile: minimal
      - name: Rust Cache
        uses: Swatinem/rust-cache@v2.2.1
      - name: Build
        run: cargo build ${{ matrix.features }}
      - name: Test
        run: cargo test ${{ matrix.features }}

The idea would be to have Nix instantiate the default shell before the run: cargo build ${{ matrix.features }}

How can I accomplish this?