Skip copying the working directory under GOPATH

What does this MR do?

Skips copying the working directory under GOPATH. This can be done safely after upgrading Go to 1.11+ (!15 (merged), !69 (merged)). Also addresses a workaround left in !71 (merged).

What are the relevant issue numbers?

n/a

Does this MR meet the acceptance criteria?

• [n/a] Changelog entry added
• [n/a] Documentation created/updated for GitLab EE, if necessary
• [n/a] Documentation created/updated for this project, if necessary
• [n/a] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [n/a] Job definition updated, if necessary
  • [n/a] Auto-DevOps template
  • [n/a] Job definition example
  • [n/a] CI Templates
• Conforms to the code review guidelines
• [n/a] Conforms to the Go guidelines
• [n/a] Security reports checked/validated by reviewer