Bootstrap pip on minimal agents before virtualenv creation

Jenkinsfile

@@ -33,6 +33,10 @@ pipeline {
             steps {
                 sh '''
                     python3 -m venv .venv || {
+                        python3 -m pip --version >/dev/null 2>&1 || {
+                            curl -fsSL https://bootstrap.pypa.io/get-pip.py -o /tmp/get-pip.py
+                            python3 /tmp/get-pip.py --user
+                        }
                         python3 -m pip install --user virtualenv
                         python3 -m virtualenv .venv
                     }