update action/cache to v4.

Author: zhuizhuhaomeng

.github/workflows/test.yml

@@ -40,7 +40,7 @@ jobs:
       run: curl -s -L http://xrl.us/cpanm > /bin/cpanm && chmod +x /bin/cpanm
 
     - name: Cache
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: |
           ~/.cpan