Fix cross-signing/SSSS reset
Fix cross-signing/SSSS reset

We re-used the old SSSS key even when resetting, meaning we prompted
the user to create a new passphrase but then ignored it and kept using
the old one.

Fixes https://github.com/vector-im/riot-web/issues/13212

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk