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_cli/vms/inspect.py

@@ -55,7 +55,7 @@ class VmConfig:
 
 
 def inspect_vm(machine: Machine) -> VmConfig:
-    data = machine.eval_nix("config.clan.core.vm.inspect")
+    data = machine.select("config.clan.core.vm.inspect")
     # HACK!
     data["flake_url"] = dataclasses.asdict(machine.flake)
     return VmConfig.from_json(data)