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/vars/generate.py

@@ -72,14 +72,20 @@ class Generator:
 
     def final_script(self) -> Path:
         assert self._machine is not None
-        final_script = self._machine.build_nix(
-            f'config.clan.core.vars.generators."{self.name}".finalScript'
+        from clan_lib.nix import nix_test_store
+
+        output = Path(
+            self._machine.select(
+                f'config.clan.core.vars.generators."{self.name}".finalScript'
+            )
         )
-        return final_script
+        if tmp_store := nix_test_store():
+            output = tmp_store.joinpath(*output.parts[1:])
+        return output
 
     def validation(self) -> str | None:
         assert self._machine is not None
-        return self._machine.eval_nix(
+        return self._machine.select(
             f'config.clan.core.vars.generators."{self.name}".validationHash'
         )