Debug: use pip list over poetry show

`poetry show` primarily describes the lockfile, and not the
poetry-managed venv. (To convince yourself of this, try `poetry show`)
in a clean checkout of synapse, before running a `poetry install`. For
debugging we mainly want to see what the virtualenv contains, so use
`pip list` instead.

For completeness, `poetry show` does have some highlighting (falling
back to warning markers when ANSI colouring is unavailable) which
indicates when the virtualenv deviates from the lockfile. Its meaning
isn't fully clear and I can't find a documentation reference for it.

Author: David Robertson

action.yml

@@ -98,5 +98,5 @@ runs:
     - name: Dump virtual environment
       run: |
         poetry env info
-        poetry show
+        poetry run pip list
       shell: bash