Add per job before_script handling for exec command

What does this MR do?

Allows for local execution of the runner via exec to recognize the before_script part of the ci configuration file

Why was this MR needed?

To fix issue for people running local builds with the runner.

Does this MR meet the acceptance criteria?

• Documentation created/updated - N/A
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #1373 (closed) and #1636 (closed)

Supercedes !298 (closed)

Mentioned in merge request !298 (closed)

Added improvement label

Milestone changed to %v1.7

Thanks @Mishkan!

Status changed to merged

Changed title: Feature/add per job before script for exec → Add per job before_script handling for exec command

Mentioned in issue #1815 (closed)