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/generate-test-vars/generate_test_vars/cli.py

@@ -63,8 +63,7 @@ class TestMachine(Machine):
     def flake_dir(self) -> Path:
         return self.test_dir
 
-    @override
-    def nix(self, attr: str) -> Any:
+    def select(self, attr: str) -> Any:
         """
         Build the machine and return the path to the result
         accepts a secret store and a facts store # TODO