CI: Install a fresh python virtual env every time (Linux)

Install a fresh python virtual environment every time to ensure
correct permissions and compatibility of the packages.
This is more robust than relying on being able to upgrade
an existing one.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>

.azurepipelines/templates/platform-build-run-steps.yml

@@ -55,7 +55,12 @@ steps:
     architecture: "x64"
   condition: ne('${{ parameters.usePythonVersion }}', '')
 
-- script: pip install -r pip-requirements.txt --upgrade
+- bash: |
+    virtualenv ~/venv
+    . ~/venv/bin/activate
+    echo "##vso[task.setvariable variable=VIRTUAL_ENV]${VIRTUAL_ENV}"
+    echo "##vso[task.prependpath]${VIRTUAL_ENV}/bin"
+    pip install -r pip-requirements.txt --upgrade
   displayName: 'Install/Upgrade pip modules'
 
 # Set default

.azurepipelines/templates/pr-gate-steps.yml

@@ -32,7 +32,12 @@ steps:
     architecture: "x64"
   condition: ne('${{ parameters.usePythonVersion }}', '')
 
-- script: pip install -r pip-requirements.txt --upgrade
+- bash: |
+    virtualenv ~/venv
+    . ~/venv/bin/activate
+    echo "##vso[task.setvariable variable=VIRTUAL_ENV]${VIRTUAL_ENV}"
+    echo "##vso[task.prependpath]${VIRTUAL_ENV}/bin"
+    pip install -r pip-requirements.txt --upgrade
   displayName: 'Install/Upgrade pip modules'
 
 # Set default