fix ubuntu for action (#516)

Author: jaeopt

.github/workflows/build.yml

@@ -20,7 +20,7 @@ on:
         required: true
 jobs:
   run_build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - name: set up JDK 8