Set alternate object directories in run_git

What does this MR do?

Set the GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable when spawning Git commands.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

During pre-receive hooks we need alternate object directories to be set correctly. It's not enough to set them on Rugged alone; this MR extends it to Git spawns as well.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Part of gitaly#955 (closed)