Add a post-message for num's known install failure

[shonfeder]

This is meant to help users workaround the known installation failure with num reported in #14646 (and the various linked issues).

The failure message was first suggested at #14646 (comment), and pointers to the configuration change needed was provided by @dbuenzli in a reply.

This PR is to propose the workaround, but I honestly don't know which versions it should be applied to or what the optimal messaging is here. All corrections and suggestions for improvement welcome.

[kit-ty-kate]

If this is a common issue why not run these before each install instead?

[shonfeder]

I'm not sure how frequent the problem is. It only hits me about once every couple of months. I'm happy with that tho, if it seems sensible! I'll wait until the additional reviewer you've tagged in weighs in :)

Thanks for the suggestion!

[dra27]

The inclusion of a message looks fine - does this issue remain macOS-specific? Can you remind why it's ocamlfind remove rather than brew remove? I had thought the problem here was the system package manager's installed copy of num, which you wouldn't normally remove using ocamlfind itself?

[shonfeder]

I hit this periodically on arch and Ubuntu, and afaik, have no installation of num other than via opam. I’m not clear on the underlying cause, unfortunately, and I am just cargo cutting the fix proposed in the linked issue :)

[mseri]

I hit the same bug both without homebrew and on linux in the past a few times, I think adding the comment is harmless and help people incurring in this in the future. Otherwise, as @kit-ty-kate suggested, let's run those commands anyway in the install step

[shonfeder]

Thanks for the better fix! :)