Use builder for dSYM steps
Use builder for dSYM steps

Summary: Refactor to use a builder to add steps. In a follow-up diff, we want to conditionally insert some steps, so this makes it much easier.

Reviewed By: blackm00n

fbshipit-source-id: d9967efd1cdaf3f8521aa5efc0329cf3869a9f1c

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