x/build/cmd/relui: tag golang.org/x repos continuously

[toothrot]

Plan: once a month (or whatever time unit)
For each x repo in dependency order (lowest first)
go get update all the deps to their latest tags
Wait for that to be green
Tag it with next version

Can probably shoehorn into existing process, maybe not running automatically at first.
0.1.0, 0.2.0, 0.3.0...

[dmitshur]

Wait for that to be green

Automating this process will be blocked on coming up with a machine-understandable criteria of what "green" means.

[gopherbot]

Change https://go.dev/cl/423036 mentions this issue: internal/workflow: clarify ownership

[gopherbot]

Change https://go.dev/cl/423037 mentions this issue: internal/workflow: add expansions

[gopherbot]

Change https://go.dev/cl/425088 mentions this issue: internal/task: start x repo tagging tasks

[gopherbot]

Change https://go.dev/cl/425089 mentions this issue: internal/task: continue x repo tagging