Build gitaly-git2go from previous gitaly version

What does this MR do?

Builds gitaly-git2go from the previous released gitaly version.

This way the binary is still available to the currently running gitaly process.

The plan is to have the new gitaly software use its own gitaly-git2go under a different name.

Related issues

Part of the plan here: gitaly#3647 (comment 599082033)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened