switch to ubuntu 20.04

Author: d99kris

.github/workflows/linux.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   linux-build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - name: Checkout
       uses: actions/checkout@v1