conformance: support the staging tests

[jku]

The conformance test GitHub action now supports testing against Sigstore staging infrastructure. Testing against staging is beneficial because when infrastructures changes, it happens first in staging: finding out ASAP is good for clients.

Enabling the support requires two things:

• conformance client test/integration/sigstore-python-conformance needs to support a new argument --staging: this can be done by replacing the script with the script from: https://github.com/sigstore/sigstore-conformance/blob/main/sigstore-python-conformance
• A second call to the conformance action (with input environment: staging) should be added to .github/workflows/conformance.yml as described in conformance README