Fix ubuntu-18.04 not found issue

Author: xumia

azurepipeline.yml

@@ -10,7 +10,7 @@ pr:
 jobs:
 - job: Build
   pool:
-    vmImage: ubuntu-18.04
+    vmImage: ubuntu-20.04
 
   variables:
     DIFF_COVER_CHECK_THRESHOLD: 80
@@ -58,7 +58,7 @@ jobs:
         set -ex
         # Install .NET CORE
         curl -sSL https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -
-        sudo apt-add-repository https://packages.microsoft.com/ubuntu/18.04/prod
+        sudo apt-add-repository https://packages.microsoft.com/ubuntu/20.04/prod
         sudo apt-get update
         sudo apt-get install -y dotnet-sdk-5.0
       displayName: "Install .NET CORE"