feat: add cache-write input for read-only cache mode

Add a 'cache-write' input (default: true) that controls whether the cache
is saved at the end of the workflow. When set to 'false', the action will
restore cached dependencies but skip saving, providing a read-only cache
mode.

This is useful for preventing cache poisoning attacks from untrusted PR
builds while still benefiting from cached dependencies.

action.yml

@@ -20,6 +20,9 @@ inputs:
     default: ${{ github.server_url == 'https://github.com' && github.token || '' }}
   cache-dependency-path:
     description: "Used to specify the path to dependency files. Supports wildcards or a list of file names for caching multiple dependencies."
+  cache-write:
+    description: "Whether to save the cache at the end of the workflow. Set to false for cache read-only mode, useful for preventing cache poisoning from untrusted PR builds."
+    default: true
   update-environment:
     description: "Set this option if you want the action to update environment variables."
     default: true

dist/cache-save/index.js

@@ -44170,6 +44170,11 @@ const cache_distributor_1 = __nccwpck_require__(92326);
 // https://github.com/actions/cache/pull/1217
 async function run(earlyExit) {
     try {
+        const cacheWriteEnabled = core.getInput('cache-write');
+        if (cacheWriteEnabled === 'false') {
+            core.info('Cache write is disabled (read-only mode). Skipping cache save.');
+            return;
+        }
         const cache = core.getInput('cache');
         if (cache) {
             await saveCache(cache);

src/cache-save.ts

@@ -9,6 +9,12 @@ import {State} from './cache-distributions/cache-distributor';
 // https://github.com/actions/cache/pull/1217
 export async function run(earlyExit?: boolean) {
   try {
+    const cacheWriteEnabled = core.getInput('cache-write');
+    if (cacheWriteEnabled === 'false') {
+      core.info('Cache write is disabled (read-only mode). Skipping cache save.');
+      return;
+    }
+
     const cache = core.getInput('cache');
     if (cache) {
       await saveCache(cache);