chore(oxcaml): CI with opam instead of nix

[art-w]

Temporarily switch back to the previous CI for OxCaml to enable testing of #12707 and #12762. The Nix CI was introduced in #12632 and should be a lot more resilient in the long term. However I wasn't able to fix it to use the published OxCaml (instead of a dev version) and the new packages from the oxcaml opam-repo that ppx_inline_test transitively relies on.

[Alizter]

That's fine by me. The nix CI wasn't meant to get in the way, but to aid reliability. When the scope of dependencies we need to pull into the test suite settles a bit we can think a bit about adapting the nix support and enabling it again.

[Alizter]

While you are on this, let's put this job directly in workflow.yml next to the other CI jobs. You can put it in the second half depending on nix-build since we aren't interested in it if dune doesn't build.

I believe the cache will be populated after this first run so best to wait for it to finish. When you push the next change we will see the cache hit too.

[shonfeder]

FWIW, I think breaking this out of the workflow.yaml, as done here makes fine sense. But don't have a strong opinion either way, and I don't think there's any need to block on that.