uniget CLI: Metadata signature verification only runs when UNIGET_IGNORE_METADATA_SIGNATURE is set

Summary The sigstore check on metadata.json is gated on the wrong side of the condition. LoadMetadata in internal/config/update.go:81 verifies the bundle only when UNIGETIGNOREMETADATASIGNATURE is non-empty, so in a normal run, where nobody sets that variable, the signature is never checked...