Add CI testing on 32-bit i386 of OCaml 4.14 and 5.3 via Docker containers

[jmid]

This PR adds CI tests on 32-bit i386 of OCaml 4.14 and 5.3 via Docker containers.

Background:
The PRNG changed in OCaml5, meaning outputs differ between 4 and 5, even when run with the same seed.
On top, OCaml supports 32-bit (on OCaml5 only via bytecode). Again, this means different output behaviour between 32-bit and 64-bit, resulting in 4 combinations overall, that QCheck supports.

Our current CI only tests on 64-bit, thus leaving a blind angle.
More than once have I discovered out-of-date 32-bit expect test files during a release to the opam-repo, which tests 32-bit in its CI. This is a bit late to catch something that ideally would come with each PR.
This PR thus offers exactly that: CI testing of 32-bit builds via Docker containers.
I've only added 2 entries - one representative (4.14 and 5.3) from each side of the PRNG split.

The addition battles a bit to make the ends meet between the prebuilt opam-images using HOME=/home/opam and an accompanying opam user with ~/.opamrc etc, and GitHub actions insisting on adding -e "HOME=/github/home" last when invoking docker 🤷

[jmid]

FTR, these can just be removed again if we move to ocaml-ci #276

[jmid]

The #331 CI runs were indicating that we were missing an opam update in the workflows.

I've added one here - to both 64-bit and 32-bit workflows.

I'll go ahead and merge, so that #331 can benefit.