Install symlink to Python 3 to work around AzP diff coverage issue (#3803)

What I did

In Azure Pipelines, the PythonScript task appears to default to expecting the Python interpreter to be present at /usr/bin/python. This task type is used as part of diff coverage reporting. However, on Ubuntu 22.04 and newer, only Python 3 is present by default, which means /usr/bin/python isn't present.

As a workaround, install the python-is-python3 package, which just adds a symlink from /usr/bin/python to /usr/bin/python3. This makes the PythonScript task happy now that it can run Python commands using the interpreter at /usr/bin/python.

When the steps that add the diff coverage task set pythonInterpreter: '/usr/bin/python3' so that PythonScript runs the script using /usr/bin/python3, or PythonScript falls back to looking for /usr/bin/python3 if /usr/bin/python is not present, then this workaround can be removed.

Author: mssonicbld

.azure-pipelines/test-docker-sonic-vs-template.yml

@@ -215,3 +215,8 @@ jobs:
     displayName: "Check ASAN reports"
     condition: eq('${{ parameters.asan }}', true)
     continueOnError: true
+
+  - script: |
+      sudo apt-get -o DPkg::Lock::Timeout=600 install -y python-is-python3
+
+    displayName: "Install temporary workaround to add a symlink to python 3"