test: run conformance against staging

[ECD5A]

Summary

Run the Sigstore conformance suite against both production and staging infrastructure.

The existing conformance wrapper already handles the action's --staging argument, but the workflow only exercised the default production environment. A two-entry matrix now gives production and staging independent jobs, keeps both environments visible in required-check output, and disables fail-fast so one failure does not hide the other environment's result.

Closes #942.

Validation:

• parsed .github/workflows/conformance.yml with PyYAML
• git diff --check
• python -m py_compile test/integration/sigstore-python-conformance

The first pushed workflow run will provide the end-to-end production and staging validation.

Release Note

None. Test infrastructure only; there is no user-facing API or CLI change.

Documentation

No documentation update is required. The workflow follows the environment: staging usage documented by sigstore/sigstore-conformance.

[jku]

This seems harmless but also is likely not as useful as it sounds:

• the environment: staging action option does not actually do very much anymore: Deprecate "environment: staging" sigstore-conformance#279 -- the ability to set trusted_root.json and signing_config.json turned out to be much more useful for the conformance test suite and the option is there mostly as a smoke test
• we already have smoke tests for staging (see e.g. staging-tests.yml) that should make noise if anything breaks