Changes in the build environment are not caught until there's a change in the MPS

[rptb1]

          Executing [proc.review.express.brainstorm](https://github.com/Ravenbrook/mps/blob/ce06e09b3031fcd9a0a40b5a0297368407c19269/procedure/review.rst#12-express-review):

1. We should investigate how we can trigger builds at intervals so that we get early warnings of platform changes even when there is not much activity in the MPS repository. This seems to be possible via a "schedule" in the workflow.

Originally posted by @rptb1 in #283 (comment)