ci: update pip

The current python action (as bumped by dependabot)
seems to still be running pip 25.2 which pip-audit flagged
for known vulnerability

Author: dannyadair

.github/workflows/pip-audit.yaml

@@ -23,6 +23,9 @@ jobs:
           echo "VIRTUAL_ENV=$PWD/.venv" >> $GITHUB_ENV
           echo "$PWD/.venv/bin" >> $GITHUB_PATH
           uv sync --dev
+      - name: Upgrade pip in venv
+        run: |
+          python -m pip install --upgrade pip
       - name: Run pip-audit
         run: |
           pip-audit