[CI] Integration failure: staging instance

[github-actions]

Staging instance failure

A scheduled test against Sigstore's staging instance has failed.

This suggests one of three conditions:

• A backwards-incompatible change in a Sigstore component;
• A regression in sigstore-python;
• A transient error.

The full CI failure can be found here:

https://github.com/sigstore/sigstore-python/actions/runs/7770231554