refactor: replace eval_nix/build_nix with machine.select()

- Remove nix(), eval_nix(), and build_nix() methods from Machine class
- Add select() method that handles machine-specific attribute prefixes
- Update all usages to use machine.select() directly
- Handle Path conversion and tmp_store logic at call sites
- This simplifies the Machine API and prepares for deployment.json removal

pkgs/clan-cli/clan_lib/tests/test_create.py

@@ -282,5 +282,5 @@ def test_clan_create_api(
     clan_dir_flake.invalidate_cache()
 
     with pytest.raises(ClanError) as exc_info:
-        machine.build_nix("config.system.build.toplevel")
+        Path(machine.select("config.system.build.toplevel"))
     assert "nixos-system-test-clan" in str(exc_info.value)