Updated CI [skip ci]

Author: ankane

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
           - ruby: "3.0"
             os: macos-latest
           - ruby: 2.7
-            os: ubuntu-18.04
+            os: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1