Remove detritus from CI image (#1384)

Safety is pedantically correct, if overzealous, and the provided image in
ubuntu-latest contains some extra pre-installed packages.  This appears to
amount to a situation where pip does not properly remove an old version of an
updated package.  In this case, `pip` itself.  So ... help it along by manually
removing the offending package.

Hopefully, this will become a non-issue shortly once the image is updated again
-- a transient error.  Meanwhile, we have a new CI cruft of our own (`rm -rf
...`)

.github/workflows/ci.yaml

@@ -49,10 +49,10 @@ jobs:
           python-version: "3.11"
       - name: install requirements
         run: |
-          python -m pip install --upgrade pip setuptools
-          python -m pip install './compute_sdk'
-          python -m pip install './compute_endpoint'
+          python -m pip install -U pip setuptools
+          python -m pip install './compute_sdk' './compute_endpoint'
           python -m pip install safety
+          rm -rf /opt/hostedtoolcache/Python/*/x64/lib/python3.*/site-packages/pip-23.{0,1,2}*
       - name: run safety check
         run: safety check