Add GitHub Actions workflow for dune pkg

[Sudha247]

This PR adds a GitHub Actions workflow for ocaml.org. This would help us catch any failures with dune pkg. Also fixed a deprecated function in Mirage_crypto_rng along the way.

[Sudha247]

To add more context: This is a preliminary check whether the project builds with dune developer preview. It seems there's some issue with depext, I'll investigate this tomorrow.

[cuihtlauac]

Wouldn't it be more prudent to begin by a local setup allowing the use of dune pkg instead of a GitHub action?

[Sudha247]

Wouldn't it be more prudent to begin by a local setup allowing the use of dune pkg instead of a GitHub action?

Indeed, and this setup was working on my machine - it was building and passing all tests. Maybe because depexts were already installed on my machine. I also discovered that it installs OCaml 4.14.x compiler version when I build it with dune pkg. Is that expected?

[sabine]

Thank you Sudha, seeing that it builds on 4.14 and how little needed to be done for that was helpful. I'm closing this in favor of #3132 where I tried to get OCaml.org to work with Dune Package Management on OCaml 5.2.0 🧡