Add `OCAMLPARAM=_,keywords=5.2` to `build-env` which allows these
packages to build in OCaml 5.3 by disabling the `effect` keyword during
the build only.

Caisar uses Re__core.replace_string directly, which got caught by a
refactoring in 1.12.0

It fails when building with ocaml-protoc-plugin.4.2.0 with 2.9.3:

File "lib/onnx/dune", line 11, characters 11-30:
11 |   (package ocaml-protoc-plugin))
                ^^^^^^^^^^^^^^^^^^^
Error: Package ocaml-protoc-plugin does not exist

Note that all subsequent versions of ocaml-proto-plugin require at least
Dune 3.2, so the simple lowerbound is sufficient - there's no version of
dependencies where this package can succesfully build with Dune 2.9.x

Uses Option.value_or_thunk which was added in base v0.15.0