Refactored the CI

[nangahamandine]

Some issues that I noticed that have been resolved include:

• Incorrect Workflow Trigger: The on section is supposed to specify events that trigger the workflow. In the present code there is the pull_request event but no action (e.g., opened, synchronize) and the push event with branches specified.Since we want it to run on every push to the "main" branch, I have defined it as such.
• Missing Matrix Variable: In the strategy section, there is the use of the matrix.os variable, but it seems like it was intended to also use the matrix.ocaml-compiler variable in the uses field of the "Use OCaml" step. I have defined this matrix variable within the matrix section.
• Improper String Escaping: In the "Use OCaml" step, there is the ocaml-compiler field with a value that seems to be a list. However, we need to specify it as a string. I've done that as well.
• In the workflow, there is the use of ${{ steps.multicore_hash.outputs.commit }} in the cache-prefix field. This is attempting to access an output variable named commit from a previous step named multicore_hash. Since the multicore_hash doesn't produce an output named commit, this results in an error.

I've resolved these problems.

[Sudha247]

Thanks for the work! #97 supersedes this, so am closing this PR.