ci: Update workflow OS to ubuntu-latest (#183)

Author: m12331

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ on:
       - '**'
 jobs:
   test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     strategy:
       matrix:
         node: [ '12', '14', '15' ]