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

@@ -17,6 +17,9 @@ inputs:
     default: true
   cache-dependency-path:
     description: 'Used to specify the path to a dependency file (e.g., go.mod, go.sum)'
+  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
   architecture:
     description: 'Target architecture for Go to use. Examples: x86, x64. Will use system architecture by default.'
 outputs:

dist/cache-save/index.js

@@ -71548,6 +71548,11 @@ process.on('uncaughtException', e => {
 function run(earlyExit) {
     return __awaiter(this, void 0, void 0, function* () {
         try {
+            const cacheWriteEnabled = core.getInput('cache-write');
+            if (cacheWriteEnabled === 'false') {
+                core.info('Cache write is disabled (read-only mode). Skipping cache save.');
+                return;
+            }
             const cacheInput = core.getBooleanInput('cache');
             if (cacheInput) {
                 yield cachePackages();

src/cache-save.ts

@@ -18,6 +18,12 @@ process.on('uncaughtException', e => {
 
 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 cacheInput = core.getBooleanInput('cache');
     if (cacheInput) {
       await cachePackages();