cmd: gpt: Preserve type GUID if enabled

If CONFIG_PARTITION_TYPE_GUID is enabled, the type GUID will be
preserved when writing out the partition string. It was already
respected when writing out partitions; this ensures that if you capture
the current partition layout and write it back (such as when renaming),
the type GUIDs are preserved.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>