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-save.ts

@@ -2,7 +2,7 @@ import * as core from '@actions/core';
 import * as cache from '@actions/cache';
 
 import {State} from './constants';
-import {getPackageManagerInfo} from './cache-utils';
+import {getPackageManagerInfo, isPackageManagerInstalled} from './cache-utils';
 
 // Catch and log any unhandled exceptions.  These exceptions can leak out of the uploadChunk method in
 // @actions/toolkit when a failed upload closes the file descriptor causing any in-process reads to
@@ -45,6 +45,17 @@ const cachePackages = async (packageManager: string) => {
     return;
   }
 
+  // Check if the package manager is installed before attempting to save cache
+  // This prevents cache save failures for package managers that may not be installed
+  const isInstalled = await isPackageManagerInstalled(packageManager);
+  if (!isInstalled) {
+    core.warning(
+      `Package manager '${packageManager}' was not found in the PATH. ` +
+        `Skipping cache save.`
+    );
+    return;
+  }
+
   if (!cachePaths.length) {
     // TODO: core.getInput has a bug - it can return undefined despite its definition (tests only?)
     //       export declare function getInput(name: string, options?: InputOptions): string;