Update checkout action to v3

Author: Philippus

.github/workflows/build.yml

@@ -4,7 +4,7 @@ jobs:
   build:
     runs-on: ubuntu-20.04
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with: