Load execution plan from the snapshot on the run when resume retrying

Summary:
Right now when you re-execute a pipeline from failure, we load an execution plan over gRPC not once, not twice, but thrice - once from the parent run to figure out the steps, once from the child run without any steps to validate that the steps are correct, and then a third time with the steps.

This diff takes the first call out of the equation by loading the execution plan directly from the snapshot.

Test Plan: BK, re-execute a pipeline from failure in dagit, verify it works and log to verify that the number of gRPC calls goes from 3 to 2

Reviewers: yuhan, prha, alangenfeld

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D9041