Bump @actions/cache to v6.1.0 - handle cache write error due to RO token

__tests__/saveOnly.test.ts

@@ -95,7 +95,11 @@ test("save with valid inputs uploads a cache", async () => {
     expect(core.setFailed).toHaveBeenCalledTimes(0);
 });
 
-test("save failing logs the warning message", async () => {
+test("save failing logs the debug message", async () => {
+    const debugMock = jest.spyOn(core, "debug");
+    const warningMock = jest.spyOn(core, "warning");
+    const failedMock = jest.spyOn(core, "setFailed");
+
     const primaryKey = "Linux-node-bb828da54c148048dd17899ba9fda624811cfb43";
 
     const inputPath = "node_modules";
@@ -103,6 +107,9 @@ test("save failing logs the warning message", async () => {
     testUtils.setInput(Inputs.Path, inputPath);
     testUtils.setInput(Inputs.UploadChunkSize, "4000000");
 
+    // A read-only / write-denied save surfaces to the action as saveCache resolving
+    // to -1; the toolkit has already logged the underlying reason. The action
+    // must not fail the job or emit its own warning.
     const cacheId = -1;
     (cache.saveCache as jest.Mock).mockResolvedValue(cacheId);
 
@@ -118,6 +125,7 @@ test("save failing logs the warning message", async () => {
         false
     );
 
-    expect(core.warning).toHaveBeenCalledTimes(1);
-    expect(core.warning).toHaveBeenCalledWith("Cache save failed.");
+    expect(debugMock).toHaveBeenCalledWith("Cache was not saved.");
+    expect(warningMock).not.toHaveBeenCalled();
+    expect(failedMock).not.toHaveBeenCalled();
 });