Explicitly using ubuntu-20.04 instead of ubuntu-latest

Author: Enmk

.github/workflows/linux.yml

@@ -12,7 +12,7 @@ env:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         compiler: [clang-6, gcc-7, gcc-8, gcc-9]