x/build/cmd/relui: merge communication tasks with the main release workflow

[dmitshur]

Now that both announcement mail and tweet tasks are ready (and, as of #47406, joined together), the next step is to append them to the end of the main release workflows that build and publish artifacts.

CC @golang/release.

[dmitshur]

This enables communication parameters to be entered at the start of workflow, and no more need to manually specify versions (those are computed during the release workflows, and available for reuse):

[gopherbot]

Change https://go.dev/cl/414034 mentions this issue: internal/workflow: make Slice method variadic

[gopherbot]

Change https://go.dev/cl/414035 mentions this issue: cmd/relui: join communication tasks in main release workflows

[gopherbot]

Change https://go.dev/cl/416218 mentions this issue: internal/relui: use workflow output with communications not merged

[gopherbot]

Change https://go.dev/cl/419515 mentions this issue: cmd/relui: add communication tasks to the end of release workflows

[gopherbot]

Change https://go.dev/cl/419516 mentions this issue: internal/relui: support communicating about a single minor release