Avoid unbound variable error when installing pip

The pip_args variable is not initialized when installing pip for
bullseye resulting in an unbound variable error when running
install_python3_pip on that debian version.
This patch fixes the issue moving pip_args inizialization to a common
place.

Change-Id: I1603c97871449b4f73e3062a705d655e9454bf33

diskimage_builder/elements/pip-and-virtualenv/install.d/pip-and-virtualenv-source-install/04-install-pip

@@ -24,6 +24,9 @@ function install_python2_pip {
     pip install $pip_args virtualenv
 }
 
+# force things to happen so our assumptions hold
+pip_args="-U --force-reinstall"
+
 if [[ $DISTRO_NAME =~ (opensuse|fedora|centos|centos7|rhel|rhel7) ]]; then
 
     # Default packages
@@ -98,9 +101,6 @@ if [[ $DISTRO_NAME =~ (opensuse|fedora|centos|centos7|rhel|rhel7) ]]; then
             ;;
     esac
 
-    # force things to happen so our assumptions hold
-    pip_args="-U --force-reinstall"
-
     # GENERAL WARNING : mixing packaged python libraries with
     # pip-installed versions always creates issues.  Upstream
     # openstack-infra uses this a lot (especially devstack) but be
@@ -270,9 +270,6 @@ else
 
         apt-get -y install $packages
 
-        # force things to happen so our assumptions hold
-        pip_args="-U --force-reinstall"
-
         install_python2_pip
         if [[ ${DIB_PYTHON_VERSION} == 3 ]]; then
             install_python3_pip