[BACKPORT 2.18.0][PLAT-8833]: Upgrade pip only during venv build time.

Summary: Upgrading pip only during venv build time rather than while activating venv

Test Plan: Jenkins pipeline

Reviewers: vbansal, sanketh

Reviewed By: sanketh

Subscribers: yugaware, jenkins-bot

Differential Revision: https://phabricator.dev.yugabyte.com/D25320

managed/devops/bin/common.sh

@@ -339,7 +339,6 @@ activate_virtualenv() {
   export SITE_PACKAGES=$(python -c "import sysconfig; print(sysconfig.get_path('purelib'))")
   PYTHON_EXECUTABLE="python"
   log "Using virtualenv python executable now."
-  run_pip install --upgrade pip > /dev/null
 
   # We unset the pythonpath to make sure we aren't looking at the global pythonpath.
   unset PYTHONPATH

managed/devops/bin/install_python_requirements.sh

@@ -72,6 +72,9 @@ if [[ $should_use_package == "1" && -f "$YB_PYTHON_MODULES_PACKAGE" ]]; then
   log "Found virtualenv package $YB_PYTHON_MODULES_PACKAGE"
   tar -xf $YB_PYTHON_MODULES_PACKAGE
 else
+
+  run_pip install --upgrade pip > /dev/null
+
   if [[ $YB_MANAGED_DEVOPS_USE_PYTHON3 == "0" ]]; then
     # looks like there is some issue with setuptools and virtualenv on python2.
     # https://github.com/pypa/virtualenv/issues/1493, adding this requirement