[CI] Integration failure: staging instance

## 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/3292043400
