qcow support build failure with ocaml 4.12

[ncopa]

Trying to build cow support on a brand new MacBook with m1 chip:

ncopa@ncopa-mbp13 hyperkit % opam install uri qcow.0.11.0 conduit.2.1.0 lwt.5.3.0 qcow-tool mirage-block-unix.2.12.0 conf-libev logs fmt mirage-unix prometheus-app

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><>  🐫 
[qcow.0.11.0] no changes from git://github.com/mirage/ocaml-qcow
[qcow-tool.0.11.0] no changes from git://github.com/mirage/ocaml-qcow

The following dependencies couldn't be met:
  - lwt → ocaml < 4.12
      base of this switch (use `--unlock-base' to force)

No solution found, exiting

ocaml was installed with brew.