Build: setting Linux runner image to ubuntu-20.04

Author: Oldes

.github/workflows/main.yml

@@ -56,7 +56,7 @@ jobs:
     strategy:
       fail-fast: true
       matrix:
-        os: [ubuntu-latest]
+        os: [ubuntu-20.04]
         #os: [ubuntu-20.04, ubuntu-18.04]
 
     runs-on: ${{ matrix.os }}