Pass along key backup for bootstrap

If we ask for the key backup key early in creating secret storage to ensure we
trust the backup, then we stash it to ensure it's available to bootstrap as well
without prompting again.

Fixes https://github.com/vector-im/riot-web/issues/12958

src/CrossSigningManager.js

@@ -185,7 +185,7 @@ export async function promptForBackupPassphrase() {
 
     const RestoreKeyBackupDialog = sdk.getComponent('dialogs.keybackup.RestoreKeyBackupDialog');
     const { finished } = Modal.createTrackedDialog('Restore Backup', '', RestoreKeyBackupDialog, {
-            showSummary: false, keyCallback: k => key = k,
+        showSummary: false, keyCallback: k => key = k,
     }, null, /* priority = */ false, /* static = */ true);
 
     const success = await finished;