Initialize staging TUF root for sigstage.dev.

[wlynch]

Summary

Follows https://github.com/sigstore/public-good-instance/blob/1023ed05b7a8cf28e6a7de73bf98dd5075d97858/playbooks/tuf.md#updating-tuf-metadata-for-staging to setup staging TUF to fix error:

========== gitsign verify ==========
Validated Certificate claims: false
WARNING: git verify-commit does not verify cert claims. Prefer using `gitsign verify` instead.
Error: ctfe public key not found for payload. Check your TUF root (see cosign initialize) or set a custom key with env var SIGSTORE_CT_LOG_PUBLIC_KEY_FILE

Release Note

NONE

Documentation

NONE