Update Workflow ubuntu version to 20.04

Author: Simon Planinschek

.github/workflows/main.yml

@@ -5,7 +5,7 @@ on:
       - 'v*'
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/create-release@v1