Disable hooks by setting an ENV var

What does this MR do?

Disable the git hooks during testing leveraging a environment variable. This is consistent with how workhorse disables the hooks for testing too.

Part of: gitaly#1590 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content