Add support for environment variables in rewrite

[danimalquackers]

Closes #280. Please be aware I used Antigravity editor (Gemini-based) to generate the code and some tests, and made slight tweaks to the output.

This PR adds support for environment variables in the rewrite parameter of a rule, for example to specify $XDG_CACHE_HOME and similar XDG variables in the rewrite path. It also adds tests to mod.rs for standard/tilde paths and environment variables requiring expansion. It preserves tildes to maintain the same behavior as before by using shellexpand::env, which only expands environment variables on evaluation, instead of shellexpand::full.

The newly added tests all pass:

$ cargo test config
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.13s
     Running unittests src/main.rs (target/debug/deps/boxxy-b622425082ce7db5)

running 6 tests
test config::tests::test_simple_cli ... ok
test config::tests::test_expand_rewrite_cli ... ok
test config::tests::test_tilde_cli ... ok
test config::tests::test_tilde_file ... ok
test config::tests::test_expand_rewrite_file ... ok
test config::tests::test_simple_file ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 4 filtered out; finished in 0.00s

And I have been able to run it successfully on my NixOS system using this home-manager configuration:

  # Redirect temp/config dirs for some apps
  programs.boxxy = {
    enable = true;

    # Use custom build with environment variable support
    package = pkgs.rustPlatform.buildRustPackage rec {
      pname = "boxxy";
      version = "0.8.5-env";
      src = pkgs.fetchFromGitHub {
        owner = "danimalquackers";
        repo = "boxxy";
        rev = "9dfa248d29efb973130ae4ee88df42261d9144b0";
        hash = "sha256-VtOtd0Cu8rJMQ3finWnWp/+ymCGmOkIA8tlA9EOWESM=";
      };
    
      # This handles the Cargo.lock dependencies
      cargoHash = "sha256-g6qSV/9UBr+UBiI1/rS3dgi0H1tbc/SDIHL3y3IVog4=";
      nativeBuildInputs = [ pkgs.pkg-config ];
    };

    rules = [
      {
        name = "Gemini CLI";
        target = "~/.gemini";
        rewrite = "$XDG_CONFIG_HOME/gemini";
      }
      {
        name = "Ansible";
        target = "~/.ansible";
        rewrite = "$XDG_DATA_HOME/ansible";
      }
      {
        name = "Terraform";
        target = "~/.terraform.d";
        rewrite = "$XDG_CACHE_HOME/terraform";
      }
    ];
  };

[queer]

Awesome stuff, thanks! I should really fix the CI configs someday...

[danimalquackers]

I'm afraid one of them might be my fault, since I installed tempfile the Cargo.lock version was bumped and it looks like the Actions runner uses an older version that doesn't support that.

[queer]

It's not on you; I've not had the energy to update them for a long time.