slack-15.0: refactor private go dep config from #367

[timvaillancourt]

Description

This PR refactors #367 by moving the git config commands to setup private-repo access (for our custom durability policy) into build.env, so the logic is more general and the GitHub token is not leaked in CI output

A separate, internal PR will occur to add GOPRIVATE to our CI environment

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes