fix(#1357): Gracefully handle missing pnpm installation during cache

This change prevents the action from failing immediately when pnpm is specified
in packageManager but not yet installed (e.g., when using corepack).

Changes:
- Add isPackageManagerInstalled() function to check if a package manager exists
- Update restoreCache to skip caching with a warning if package manager not found
- Update cachePackages to skip cache save with a warning if package manager not found
- This allows workflows to continue instead of failing
- Users can either install pnpm first or disable caching with package-manager-cache: false

Fixes #1357
Related: https://github.com/actions/setup-node/issues/1357

src/cache-utils.ts

@@ -110,6 +110,30 @@ export const getPackageManagerInfo = async (packageManager: string) => {
   }
 };
 
+/**
+ * Checks if a package manager is installed and available on the PATH
+ * This helps prevent cache failures when a package manager is specified
+ * but not yet installed (e.g., pnpm via corepack)
+ * See: https://github.com/actions/setup-node/issues/1357
+ */
+export const isPackageManagerInstalled = async (
+  packageManager: string
+): Promise<boolean> => {
+  try {
+    const {exitCode} = await exec.getExecOutput(
+      `${packageManager} --version`,
+      undefined,
+      {
+        ignoreReturnCode: true,
+        silent: true
+      }
+    );
+    return exitCode === 0;
+  } catch {
+    return false;
+  }
+};
+
 /**
  * getProjectDirectoriesFromCacheDependencyPath is called twice during `restoreCache`
  *  - first through `getCacheDirectories`