Let's update to latest pip for installer tests

.azure-pipelines/templates/jobs/packaging-jobs.yml

@@ -71,18 +71,23 @@ jobs:
         displayName: Check Powershell 5.x is used in vs2017-win2016
       - task: UsePythonVersion@0
         inputs:
-          versionSpec: 3.7
-          architecture: x86
+          versionSpec: 3.8
           addToPath: true
       - task: DownloadPipelineArtifact@2
         inputs:
           artifact: windows-installer
           path: $(Build.SourcesDirectory)/bin
         displayName: Retrieve Windows installer
+      # pip 9.0 provided by pipstrap is not able to resolve properly the pywin32 dependency
+      # required by certbot-ci: as a temporary workaround until pipstrap is updated, we install
+      # a recent version of pip, but we also to disable the isolated feature as described in
+      # https://github.com/certbot/certbot/issues/8256
       - script: |
           py -3 -m venv venv
-          venv\Scripts\python letsencrypt-auto-source\pieces\pipstrap.py
+          venv\Scripts\python -m pip install pip==20.2.3 setuptools==50.3.0 wheel==0.35.1
           venv\Scripts\python tools\pip_install.py -e certbot-ci
+        env:
+          PIP_NO_BUILD_ISOLATION: no
         displayName: Prepare Certbot-CI
       - script: |
           set PATH=%ProgramFiles(x86)%\Certbot\bin;%PATH%