Fixes ci to use ubuntu-20.04

Author: xoofx

.github/workflows/CI.yml

@@ -10,7 +10,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
     - name: Checkout

src/LibObjectFile.Tests/LinuxUtil.cs

@@ -17,7 +17,7 @@ public static string ReadElf(string file, string arguments = "-W -a")
             return RunLinuxExe("readelf", $"{file} {arguments}");
         }
 
-        public static string RunLinuxExe(string exe, string arguments, string distribution = "Ubuntu-18.04")
+        public static string RunLinuxExe(string exe, string arguments, string distribution = "Ubuntu-20.04")
         {
             if (exe == null) throw new ArgumentNullException(nameof(exe));
             if (arguments == null) throw new ArgumentNullException(nameof(arguments));