[PETOSS-980] Fix twine install escaping the venv in publish workflow

sudo pip install twine ran outside the activated venv and tried to
upgrade the system's apt-installed packaging package, which has no
pip RECORD metadata and can't be uninstalled, failing the Install
dependencies step. Install twine via the venv's pip instead.

Author: Chris Mitchell

.github/workflows/publish.yml

@@ -34,7 +34,7 @@ jobs:
                 python -m venv venv
                 source venv/bin/activate
                 pip install --upgrade pip
-                sudo pip install twine
+                pip install twine
               working-directory: xero-python
 
             - name: Fetch Latest release number