[ci] Fix pipeline issue caused by sonic-slave-* change. (#2709) (#2880)

* [ci] Fix pipeline issue caused by sonic-slave-* change. (#2709)

What I did
These 3 packages maybe purged by default. Do not block pipeline.
Download deb/whl packages only to accelerate download process.
How I did it
How to verify it

* test

azure-pipelines.yml

@@ -29,6 +29,14 @@ stages:
       image: sonicdev-microsoft.azurecr.io:443/sonic-slave-buster:202111
 
     steps:
+    - script: |
+        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/debian/10/prod
+        sudo apt-get update
+        sudo apt-get install -y dotnet-sdk-5.0
+      displayName: "Install .NET CORE"
     - task: DownloadPipelineArtifact@2
       inputs:
         source: specific
@@ -44,7 +52,7 @@ stages:
 
     - script: |
         set -xe
-        sudo apt-get -y purge libhiredis-dev libnl-3-dev libnl-route-3-dev
+        sudo apt-get -y purge libhiredis-dev libnl-3-dev libnl-route-3-dev || true
         sudo dpkg -i libnl-3-200_*.deb
         sudo dpkg -i libnl-genl-3-200_*.deb
         sudo dpkg -i libnl-route-3-200_*.deb
@@ -84,15 +92,6 @@ stages:
       workingDirectory: $(Pipeline.Workspace)/target/python-wheels/buster
       displayName: 'Install Python dependencies'
 
-    - script: |
-        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/debian/10/prod
-        sudo apt-get update
-        sudo apt-get install -y dotnet-sdk-5.0
-      displayName: "Install .NET CORE"
-
     - script: |
         python3 setup.py test
       displayName: 'Test Python 3'