pallet-scheduler: Put back postponed tasks into the agenda (#7790)

Right now `pallet-scheduler` is not putting back postponed tasks into
the agenda when the early weight check is failing. This pull request
ensures that these tasks are put back into the agenda and are not just
"lost".

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Alexandre R. Baldé <alexandre.balde@parity.io>