Unset OPAM_SWITCH_PREFIX when using make cold

[kit-ty-kate]

Otherwise if for example z3 is installed in your local switch make cold fails with:

Downloading vendored source dependencies...
done
Extracting vendored source dependencies in src_ext/... done
env PATH="`pwd`/bootstrap/ocaml/bin:$PATH" CAML_LD_LIBRARY_PATH= OCAML_TOPLEVEL_PATH= make
make[1]: Entering directory '/home/kit_ty_kate/work/opam'
src_ext/dune-local/dune.exe build --profile=release --root .  --promote-install-files -- opam-installer.install opam.install
File "src/solver/opamBuiltinZ3.ml", line 1:
Error: /home/kit_ty_kate/.opam/default/lib/z3/z3.cmi
       is not a compiled interface for this version of OCaml.
It seems to be for a newer version of OCaml.
make[1]: *** [Makefile:146: build-opam] Error 1
make[1]: Leaving directory '/home/kit_ty_kate/work/opam'
make: *** [Makefile:287: cold] Error 2

[rjbou]

Thanks!