MDEV-15680 xb_aws_key_management fails in buildbot.

aws_key_management needs current directory to be datadir during
initalization, it scans current directory for encrypted keys.

Fix is to ensure, that plugin initialization in mariabackup happens
after the call to my_setwd(mysql_real_data_home).

extra/mariabackup/xtrabackup.cc

@@ -3804,7 +3804,7 @@ xtrabackup_backup_func()
 		return(false);
 	}
 	msg("mariabackup: cd to %s\n", mysql_real_data_home);
-
+	encryption_plugin_backup_init(mysql_connection);
 	msg("mariabackup: open files limit requested %u, set to %u\n",
 	    (uint) xb_open_files_limit,
 	    xb_set_max_open_files(xb_open_files_limit));
@@ -5277,7 +5277,6 @@ xb_init()
 			return(false);
 		}
 
-		encryption_plugin_backup_init(mysql_connection);
 		history_start_time = time(NULL);
 
 	}