x/build/cmd/relui: relui should also announce to a mastodon account

[dr2chase]

Russ pinged me about this late last year; this is a bug to track that.
We have an account, it is active or nearly so.

Relui needs to post to this account to announce new releases.

[gopherbot]

Change https://go.dev/cl/554375 mentions this issue: internal/task, relui: add posting to Mastodon