[PETOSS-980] Persist venv on PATH so twine resolves in later steps

Each workflow run: block is a fresh shell, so the venv activated in
Install dependencies didn't carry over to the Publish to PyPi step.
Append the venv's bin dir to GITHUB_PATH so twine keeps resolving
there without needing to escape the venv via sudo.

Verified locally with act (catthehacker/ubuntu:act-latest, which has
the same apt-installed python3-packaging 24.0 as the real runner):
the old sudo pip install step fails the same way, the new step
installs twine cleanly inside the venv, twine --version resolves
correctly in a later step with no venv activation, and the apt
packaging package is left untouched.

Author: Chris Mitchell

.github/workflows/publish.yml

@@ -35,6 +35,7 @@ jobs:
                 source venv/bin/activate
                 pip install --upgrade pip
                 pip install twine
+                echo "$PWD/venv/bin" >> "$GITHUB_PATH"
               working-directory: xero-python
 
             - name: Fetch Latest release number