dev/ci: set GOPROXY everywhere

Created by: bobheadxi

Follow-up to https://github.com/sourcegraph/sourcegraph/pull/28900 - noticed that we can just set this for all steps, rather than manually provide it for specific ones. Also adds a link to the proxy deployment - this was provided in a PR comment but not in the actual code (https://github.com/sourcegraph/sourcegraph/pull/28900#discussion_r771394216)

Just for fun, writing reading through this stuff for a personal blog thing : )

Test plan

main-dry-run: https://buildkite.com/sourcegraph/sourcegraph/builds/132991