Get rid of calls to opam when using opam

[kit-ty-kate]

No description provided.

[mato]

@kit-ty-kate Sorry, lost track of what's happening here. As I understand it, ocaml-freestanding will work fine with OCaml 4.11 if I just update the availability, and indeed I just tested this, so I'll make that change in a separate PR to master.

The other changes here may or may not be required for OPAM 2.1?

[kit-ty-kate]

Sorry for the lack of description on this PR. The previous version of ocaml-freestanding is already installable with OCaml 4.11 (see ocaml/opam-repository#16976). The only thing that this PR does is to make the build the tinyest bit faster and generally easier to install without opam.

OPAM 2.1 does not have to do anything wrt that, it was just a bug that has been fixed a while ago, but in my opinion it highlighted parts of packages that could be improved by making them the tiniest bit faster and more portable.

[mato]

Ok, I get what this is supposed to do, but am going to close it for now; it introduces dependencies on a $PREFIX in the environment in ways that are not quite obvious, and in any case opam config var prefix takes about 0.013s to execute on my system.