Though the mammary glands could be a past adaptation rendered useless (like the human tailbone), since it is an old mutated trait, seen in all Argonians through all TES games, its unlikely. They could also be some sort of mating bumps, although this is impractical since all sub species (game wise) have these and would be a later adaptation, on a regional level. If they truly produce food for offspring, they would most likely have few children (few inner eggs, those eggs however are not nourished by a yolk, but a respiratory gas exchange) since Argonians are ovoviviparous.
Assuming there is some sort of nourishment in the bumps, it wouldn't be milk. It would probably be some sort of Hist sap concoction. This is a huge step in evolution, being able to feed your own young, although like stated above, you usually have fewer children. but a few sentences from a dev could change it all.