Install dotnet core to fix python gcov warning for code covery color bar showing (#215)

**- What I did**
fix python gcov warning "Please install dotnet core to enable automatic generation of Html report"
**- How I did it**
install dotnet core

Author: pettershao-ragilenetworks

azure-pipelines.yml

@@ -44,7 +44,7 @@ stages:
 
     - script: |
         set -ex
-
+        sudo apt-get -y purge libhiredis-dev libnl-3-dev libnl-route-3-dev
         sudo dpkg -i ../target/debs/buster/{libswsscommon_1.0.0_amd64.deb,python3-swsscommon_1.0.0_amd64.deb,libnl-3-200_*.deb,libnl-genl-3-200_*.deb,libnl-nf-3-200_*.deb,libnl-route-3-200_*.deb,libhiredis0.14_*.deb}
         sudo python3 -m pip install ../target/python-wheels/swsssdk*-py3-*.whl
         sudo python3 -m pip install ../target/python-wheels/sonic_py_common-1.0-py3-none-any.whl
@@ -59,6 +59,15 @@ stages:
         python3 setup.py test
       displayName: "Unit tests"
 
+    - 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: PublishTestResults@2
       inputs:
         testResultsFiles: '$(System.DefaultWorkingDirectory)/test-results.xml'