fix: pass the installer's own lint gate

install.py held a local it never read and, since the registry path
rules, a loop variable shadowing the platform module, so the py38
ruff gate the installer is held to failed. Both are fixed, and the
suite reads the two rules from the syntax tree since CI has no ruff.

tests/test_install.py

@@ -1151,6 +1151,52 @@ class InstallerStaysOnPython38AndTheStdlib(unittest.TestCase):
         self.assertEqual(used, [])
 
 
+    def test_no_local_is_assigned_and_never_read(self):
+        """ruff --select F in py38 mode is the installer's lint gate; an
+        unread `appdata` in _os_default_dests made it exit 1. The suite has
+        no ruff, so the rule is read from the syntax tree."""
+        import ast  # noqa: PLC0415
+
+        tree = ast.parse((Path(__file__).resolve().parents[1] / "install.py").read_text())
+        unread = []
+        for func in ast.walk(tree):
+            if not isinstance(func, (ast.FunctionDef, ast.AsyncFunctionDef)):
+                continue
+            stored, loaded = {}, set()
+            for node in ast.walk(func):
+                if isinstance(node, ast.Name):
+                    if isinstance(node.ctx, ast.Store):
+                        stored.setdefault(node.id, node.lineno)
+                    else:
+                        loaded.add(node.id)
+            nested = {
+                n.id for inner in ast.walk(func) if inner is not func
+                and isinstance(inner, (ast.FunctionDef, ast.Lambda))
+                for n in ast.walk(inner) if isinstance(n, ast.Name)
+            }
+            unread += [
+                f"{func.name}:{line} {name}" for name, line in stored.items()
+                if name not in loaded | nested and not name.startswith("_")
+                and not any(
+                    isinstance(g, (ast.Global, ast.Nonlocal)) and name in g.names
+                    for g in ast.walk(func)
+                )
+            ]
+        self.assertEqual(unread, [])
+        # Nor a module rebound: a loop variable named `platform` shadowed the
+        # platform module in _detect_by_path (F402).
+        modules = {
+            alias.asname or alias.name.split(".")[0]
+            for node in tree.body if isinstance(node, ast.Import)
+            for alias in node.names
+        }
+        rebound = sorted(
+            f"{node.id}:{node.lineno}" for node in ast.walk(tree)
+            if isinstance(node, ast.Name) and isinstance(node.ctx, ast.Store)
+            and node.id in modules
+        )
+        self.assertEqual(rebound, [])
+
 class ARefusedCopyIsAnError(unittest.TestCase):
     """A PermissionError on a standalone copy was counted as "dir not found"
     and the run ended on Done with exit code 0."""