prepare-zanata-client: upgrade pip in venv

Make sure the pip in the venv is upgraded past the system-version so
it can install all the latest requirements.

Change-Id: I601aca1507d16912e08ee9aba8842bc205ea2a91

roles/prepare-zanata-client/files/common_translation_update.sh

@@ -102,6 +102,10 @@ function finish {
 function setup_venv {
     if [ -d ~/.venv ] ; then
         source ~/.venv/bin/activate
+        # Ensure this venv is running the latest pip; if this has come
+        # from the system pip we might have a bionic-era pip that
+        # can't build modern things.
+        pip install --upgrade pip
     else
         # Ensure ~/.local/bin is in the path
         export PATH=~/.local/bin:$PATH