fix: address changes done to Sigstore TUF repository

The Sigstore project changed the internals of its TUF repository, which
broke sigstore-rs.

This commit updates to the latest version of sigstore-rs. The code
changes have been caused by the massive changes done by sigstore-rs.

Fixes kubewarden/kwctl#753

Signed-off-by: Flavio Castelli <fcastelli@suse.com>

Author: flavio