Update checkout action to v3

Author: xuanxu

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         - ruby-version: 'head'
           failure-allowed: true
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v3
     - name: Set up Ruby
       uses: ruby/setup-ruby@v1
       with: