ocamlPackages.findlib: don't wait configurePhase to set OCAMLFIND_DESTDIR

[symphorien]

this fixes ocamlfind printconf destdir inside nix-shell

fixes #152028

the regression was introduced by a3ab43d which was meant to fix building ocamlPackages.ocamlbuild with strictDeps. This commit keeps ocamlbuild building.

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[trofi]

Bisect says this change broke ocamlPackages.ocaml_libvirt in staging:

$ nix build -f. ocamlPackages.ocaml_libvirt -L
...
ocaml-libvirt> ocamlfind install  -ldconf ignore libvirt \
ocaml-libvirt>   ../META *.so *.a *.cma *.cmx *.cmxa *.cmi *.mli
ocaml-libvirt> ocamlfind: Bad configuration: Cannot mkdir /nix/store/iw3q68k4il2zqch2z17ifxygqcs5sjgd-ocaml-libvirt-0.6.1.5/lib/ocaml/4.12.0/site-lib/libvirt because a path component does not exist or is not a directory
ocaml-libvirt> make[1]: *** [Makefile:134: install-opt] Error 2
ocaml-libvirt> make[1]: Leaving directory '/build/source/libvirt'
ocaml-libvirt> make: *** [Makefile:31: install-opt] Error 1

[trofi]

Probably because ocamlPackages.ocaml_libvirt overrides pre/post hooks with phase overrides:

  buildPhase = "make all opt CPPFLAGS=-Wno-error";
  installPhase = "make install-opt";

[trofi]

Proposed fix as #156825