CI(update-smoke): drop cache: 'pip' to avoid fatal post-step

studio-update-smoke runs install.sh + unsloth studio update --local.
Both go through uv and never write to ~/.cache/pip. setup-python's
post-step then fails with:

  ##[error]Cache folder path is retrieved for pip but doesn't exist
  on disk: /home/runner/.cache/pip. This likely indicates that
  there are no dependencies to cache.

Failing the whole job at cleanup time even though all real test
steps passed (install + 2 updates + boot Studio + /api/health).
Remove the cache directive.

.github/workflows/studio-update-smoke.yml

@@ -56,7 +56,11 @@ jobs:
       - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0
         with:
           python-version: '3.12'
-          cache: 'pip'
+          # Don't cache pip: this job runs `bash install.sh` and
+          # `unsloth studio update --local` which both go through
+          # `uv` and never populate ~/.cache/pip. setup-python's
+          # post-step then fatal-errors with "Cache folder path is
+          # retrieved for pip but doesn't exist on disk".
 
       - name: Install Studio (--local, --no-torch)
         # Pass the workflow token so the llama.cpp prebuilt installer's