Delete missing/multiple/duplicate caches in "pre-commit" job

"gh cache list" may occasionally give zero results; multiple results; or
multiple results with the same cache key. Allow all of these to succeed
with exit code 0.

.github/workflows/pytest.yaml

@@ -113,7 +113,7 @@ jobs:
 
     - name: Force recreation of pre-commit virtual environment for mypy
       if: github.event_name == 'schedule'  # Comment this line to run on a PR
-      run: gh cache delete $(gh cache list -L 999 | cut -f2 | grep pre-commit)
+      run: gh cache list -L 999 | cut -f2 | grep pre-commit | xargs -I{} 'gh cache delete "{}" || true'
       env: { GH_TOKEN: "${{ github.token }}" }
 
     - uses: pre-commit/action@v3.0.0