Repair cached Python directory junctions on Windows (#1122)

With `cache-python: true`, a Windows runner can report a cache hit while
extracting the cache incorrectly. When `@actions/cache`'s tar extracts
uv's minor-version junction before the patch directory it points to, it
creates a file symlink. Resolving that link later raises `[WinError 5]
Access is denied`.

This PR fixes things up after restoring the Python cache, recreating the
junction if resolving the link fails with access denied and its recorded
target is a matching, accessible CPython installation in the same
directory.

This is a temporary workaround until
https://github.com/actions/toolkit/issues/2562 gets resolved and
`@actions/cache` itself becomes resilient to these situations.

I tested this by restoring the same archive on two fresh `windows-2022`
runners. The original run passed because resolving the minor-version
link raised the expected WinError 5. With the patch, `uv sync --offline
--managed-python --no-python-downloads` created a virtual environment
whose `home` in `pyvenv.cfg` resolved to the cached CPython 3.10.22
installation. [Original
run](https://github.com/astral-sh/ruff/actions/runs/41469344610/job/125358623949#step:9:1)
· [Patched
run](https://github.com/astral-sh/ruff/actions/runs/41469344610/job/125358623835#step:10:1).

__tests__/cache/restore-cache.test.ts

@@ -1,3 +1,4 @@
+import { fileURLToPath } from "node:url";
 import { beforeEach, describe, expect, it, jest } from "@jest/globals";
 import { createSetupInputs } from "../helpers/setup-inputs";
 
@@ -48,6 +49,42 @@ beforeEach(() => {
 });
 
 describe("restoreCache", () => {
+  it.each([
+    ["win32", false],
+    ["linux", true],
+  ] as const)(
+    "keeps the matched Python cache key on %s when the install directory cannot be read",
+    async (platform, pythonCacheHit) => {
+      const inputs = createSetupInputs({
+        cachePython: true,
+        // A file at the install directory path makes readdir fail.
+        pythonDir: fileURLToPath(import.meta.url),
+        restoreCache: true,
+      });
+      mockRestoreCache
+        .mockReturnValueOnce("uv-key")
+        .mockReturnValueOnce("python-key");
+
+      const originalPlatform = process.platform;
+      Object.defineProperty(process, "platform", { value: platform });
+      try {
+        await restoreCache(inputs);
+      } finally {
+        Object.defineProperty(process, "platform", { value: originalPlatform });
+      }
+
+      expect(mockSetOutput).toHaveBeenCalledWith("cache-hit", true);
+      expect(mockSetOutput).toHaveBeenCalledWith(
+        "python-cache-hit",
+        pythonCacheHit,
+      );
+      expect(mockSaveState).toHaveBeenCalledWith(
+        "python-cache-matched-key",
+        "python-key",
+      );
+    },
+  );
+
   it("encodes Python version ranges before adding them to the cache key", async () => {
     await restoreCache(createSetupInputs(), ">3.10.11,<3.11");