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BRB (from memory) says that anytime 2 actions happen at the same time, the player whose turn it is get to decide which action happens first.
So in this case, the player would obviously choose to resolve his Nova first.
As a side note, this probably would not matter since all the hits are allocated at the same time anyway. It makes sense to resolve them 1 unit at a time, but at the moment the Nova/Blast occurred, it still hit all units in range. So you would still resolve the hits even if you move away.
You could resolve a shooting attack prior to those hits being resolved though, as Soulburst actions happen "immediately"
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