figure out how to test on a 32-bit architecture

oss-fuzz has uncovered panics in 32-bit builds, but we do not currently run our tests on any 32-bit architectures in CI which means we cannot currently add (useful) test cases to reproduce these failures.

There are no 32-bit runners available through github actions, (https://github.com/actions/runner/issues/1181) so we will either need to configure a custom runner or else do something like setup a docker container or similar in CI, and run our tests in that.

Review the existing GitHub Actions workflows and test commands, then compare a custom runner with a Docker-based 32-bit job. Done means CI runs the test suite on a 32-bit architecture and can support regression cases for the oss-fuzz panics.

Written by the indexing model from the issue text.