Make pr-run-mode="upload" handle multiple tags

[duckinator]

With #1579, dist plan can handle workspaces having projects with different versions.

However, that change means workflows generated with pr-run-mode="upload" will fail on the build job, so #1587 made it fail in the plan job for better UX.

The proper solution would be to have workflows generated with pr-run-mode="upload" run dist build for every discovered tag, and undo the changes made in #1587.

This may involve adding a subcommand which lists all known tags. #1595 includes code that does this.

(Split this off of #1554.)