[github] fix opam cache
[github] fix opam cache

Summary:
Fix the path to ~/.opam, and move *before* the OCaml setup so that OCaml
is cached too.

I'm not convinced it ever worked, as `${HOME}` is not interpreted by the
action.

Reviewed By: martintrojer

Differential Revision: D24160822

fbshipit-source-id: b1e0fcd59

facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,facebook/infer