dev/ci: only apply GOPROXY on non-baremetal agents
Created by: bobheadxi
Test plan
main-dry-run 1: https://buildkite.com/sourcegraph/sourcegraph/builds/133192
main-dry-run 2: https://buildkite.com/sourcegraph/sourcegraph/builds/133205
Created by: bobheadxi
main-dry-run 1: https://buildkite.com/sourcegraph/sourcegraph/builds/133192
main-dry-run 2: https://buildkite.com/sourcegraph/sourcegraph/builds/133205