x/tools: add a relui workflow to automate goimports stdlib regeneration

[findleyr]

#38706 is a task that must be completed by the tools team for each Go release. It involves a minor edit and running go generate ./internal/imports within x/tools. In other words, it is an automation that should be straightforward to implement in the release team's relui workflow system, by defining a new task that creates the requisite CL.

Since we're interested in using relui for other automation, this seems like a good place to start.

CC @golang/tools-team

[gopherbot]

Change https://go.dev/cl/436515 mentions this issue: internal/task: add a relui workflow to automate goimports stdlib regeneration

[gopherbot]

Change https://go.dev/cl/442157 mentions this issue: internal/relui: extend VersionTasks feature to automate goimports stdlib regeneration

[gopherbot]

Change https://go.dev/cl/448715 mentions this issue: internal/relui: add output that registers CreateUpdateStdlibIndexCL

[gopherbot]

Change https://go.dev/cl/464535 mentions this issue: internal/task: skip CreateUpdateStdlibIndexCL

[gopherbot]

Change https://go.dev/cl/464815 mentions this issue: cmd/relui: fix up goimports regeneration task, add one-off workflow

[gopherbot]

Change https://go.dev/cl/464915 mentions this issue: cmd/relui: add await step for goimports CL, remove ad-hoc workflow