Fix broken devShell by adding flake-utils support for multiple systems

[Copilot]

The Nix flake was hardcoded to only support x86_64-linux systems, making it incompatible with other architectures like macOS (both Intel and Apple Silicon) and ARM Linux systems.

Before:

{
  description = "flake to setup the dev env required for Tandem";
  inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
  outputs = { self, nixpkgs }:
    let
      system = "x86_64-linux";  # Hardcoded system limitation
      pkgs = import nixpkgs {
        inherit system;
        config.allowUnfree = true;
      };
    in
    {
      devShells.${system}.default = # Only works on x86_64-linux
        with pkgs;
        mkShell { ... };
    };
}

After:

{
  description = "flake to setup the dev env required for Tandem";
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    flake-utils.url = "github:numtide/flake-utils";  # Added flake-utils
  };
  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:  # Support all default systems
      let
        pkgs = import nixpkgs {
          inherit system;
          config.allowUnfree = true;
        };
      in
      {
        devShells.default =  # Works on all supported architectures
          with pkgs;
          mkShell { ... };
      });
}

This change enables the development environment to work across all platforms supported by Nix:

• x86_64-linux (Intel/AMD Linux)
• x86_64-darwin (Intel macOS)
• aarch64-linux (ARM Linux)
• aarch64-darwin (Apple Silicon macOS)

All existing development dependencies (go, gopls, gotools, vagrant, sqlc, goreleaser) and shell configuration remain unchanged.

Fixes #34.

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[yyovil]

We don't want vagrant on aarch64 and remove source tandem.env as it should be the .envrc file