…ead_encryption()

Condition 1 checks use_dumped_tablespace_keys without !srv_backup_mode,
causing assertion failure in xb_set_encryption() when --transition-key
is used and undo encryption keys exist in redo log. During backup the
encryption_info map is empty (keys are being collected, not loaded),
so xb_fetch_tablespace_key() returns false and ut_a(found) fires.

Add !srv_backup_mode to match Condition 2 and all other call sites.