Bump the Solo5 cross job to OCaml 5.4.1

[samoht]

ocaml-solo5 now requires ocaml = 5.4.1, so the Cross-Solo5 job no longer resolves on 5.3.0:

[ERROR] No solution for ocaml-solo5:
  - (invariant) → ocaml-base-compiler = 5.3.0
  - ocaml-solo5 → ocaml = 5.4.1 → ocaml-base-compiler >= 5.4.1~

This PR bumps the job to 5.4.1. It unblocks the cross CI for #29 and #30.

[hannesm]

Thanks for your effort. #33 includes this fix, and makes the CI going green.