Deploy workflow: upgrade pip before installing tox

This is a flailing attempt to prevent the apparently intermittent
failure which happened in run 51, attempts [1][] and [2][].

The specific error was "`Error while finding module specification for 'pip' (AttributeError:
module '__main__' has no attribute '__file__')`" while Lektor was
running pip to install a local package.

[1]: https://github.com/lektor/lektor-website/actions/runs/1661330194/attempts/1
[2]: https://github.com/lektor/lektor-website/actions/runs/1661330194/attempts/2

.github/workflows/deploy.yml

@@ -23,7 +23,10 @@ jobs:
       - uses: actions/setup-node@v2
       - uses: actions/setup-python@v2
 
-      - run: pip install tox
+      - name: Install tox
+        run: |
+          python -m pip install --upgrade pip setuptools
+          python -m pip install tox
 
       - run: echo DEPLOY='${{ env.DEPLOY }}'
       - name: Run tox to build site