Change execution_id length to be shorter

[aviruthen]

Issue #, if available:
Issue # 5269: Execution ID has different length and characters in SageMaker Local Mode vs remote execution. Here is the link to this issue: #5269

Description of changes:
Changed execution ID to be shorter, from the full length of the uuid4() to just the first 12 characters.

Testing done:
Ran unit tests locally and tested same test file that user who raised GitHub issue provided to demonstrate issue (and verified that the change fixed this issue).

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