x/build/cmd/coordinator: update build.golang.org dashboard during LUCI migration

[dmitshur]

We're getting close to the post-submit phase of the migration to LUCI, with many LUCI builders ported to https://ci.chromium.org/p/golang and ready to replace the previous coordinator-powered counterparts. This is the tracking issue to update the build.golang.org dashboard during the transition.

The plan is to gradually start displaying LUCI build results on the existing build.golang.org dashboard, and work towards removing duplicate builders from coordinator over time. There are some known issues in the LUCI post-submit UI that we'll be tracking separately (#66036), so fully moving to that UI is out of scope for this issue.

CC @golang/release.

[gopherbot]

Change https://go.dev/cl/567499 mentions this issue: cmd/coordinator/internal/legacydash: refactor package-scoped variables

[gopherbot]

Change https://go.dev/cl/567498 mentions this issue: cmd/coordinator, perf/app: prepare a placeholder banner

[gopherbot]

Change https://go.dev/cl/567496 mentions this issue: cmd/coordinator: disable Kubernetes mode in local "dev" mode

[gopherbot]

Change https://go.dev/cl/567497 mentions this issue: cmd/coordinator: rewrite scheme-less URLs and redirects in dev mode

[gopherbot]

Change https://go.dev/cl/567578 mentions this issue: cmd/coordinator: display LUCI build results on build dashboards

[gopherbot]

Change https://go.dev/cl/567577 mentions this issue: WIP: cmd/coordinator/internal/legacydash: add LUCI build result support

[gopherbot]

Change https://go.dev/cl/567575 mentions this issue: WIP: cmd/coordinator/internal/lucipoll: add package

[gopherbot]

Change https://go.dev/cl/567576 mentions this issue: WIP: cmd/coordinator/internal/dashboard: add LUCI build result support