Update CI to Ubuntu 20.04

.github/workflows/build.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 jobs:
   linux:
     name: Linux
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Install Dependencies