Set GOPROXY to speed up Go module downloads
I noticed two things while waiting for a GDK build on another MR:
- A lot of time spent downloading Go modules
- Go module downloads sometimes failing, causing the whole build to abort
The Go project has proxy infrastructure (similar to how Bundler has a CDN) to solve these two problems. It will be on by default in a later Go version but it isn't as of Go 1.12. By setting GOPROXY in our Makefile, we should get all GitLab Go components to actually use the proxy.