wic: use part_name when defined

wic: use part_name when defined

So far part.label has been used to define GPT partition label even if
part.part_name was defined.
Fix the code to use part.part_name whenever available, as it makes sense
to have a GPT partition label which is different from the contained
filesystem label.

Signed-off-by: Dit Kozmaj <dit.kozmaj@kynetics.com>
Signed-off-by: Diego Rondini <diego.rondini@kynetics.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>