use latest ubuntu runner

Author: anthonyshchang

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: Test
 on: [push]
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - name: Use Node.js 12.8