Updated agent pool from windows to linux (#21651)

Author: vidai-msft

.azure-pipelines/test-coverage.yml

@@ -1,8 +1,8 @@
 parameters:
-- name: windowsImage
-  displayName: Windows Image
+- name: poolName
+  displayName: Pool Name
   type: string
-  default: windows-2019
+  default: pool-ubuntu-2004
 - name: dotnet_sdk_3_1
   displayName: .NET Core 3.1
   default: 3.1.x
@@ -30,7 +30,7 @@ jobs:
 - job: TestCoverage
   timeoutInMinutes: 600
   pool:
-    vmImage: ${{ parameters.windowsImage }}
+    name: ${{ parameters.poolName }}
 
   steps:
   - task: UseDotNet@2