Test building the historical spec more frequently than release cadence

to avoid issues where outdated hugo versions are forgotten and only tested at release time: https://github.com/matrix-org/matrix-spec/commit/4e51970c55715525b2f405a78fe41422521ceb24

Start by reading the linked commit (4e51970c55715525b2f405a78fe41422521ceb24) and the repository's existing historical-spec build entry point. Determine how builds are currently tied to the release cadence, then define a more frequent check that exercises older Hugo versions. Done means outdated Hugo versions are detected before release time.

Written by the indexing model from the issue text.