Add a 32 bit system to the CI

[perturbing]

Currently our CI only builds on 64-bit systems. We should also run CI on 32-bit targets to catch hidden assumptions. See this comment for context.

[perturbing]

@angerman

In cardano-base we have Cardano.Crypto.PackedBytes, which uses CPP to select different implementations depending on WORD_SIZE_IN_BITS (64 vs 32). Right now our CI only exercises the 64-bit branch, so any 32-bit-specific issues/assumptions could slip through.

Would it be feasible to add a 32-bit job on Hydra by cross-compiling (e.g., to a musl32 target) using haskell.nix?

[angerman]

Yes. @hamishmack should be able to add a 32bit cross target.