mirage/mirage-dev: PR #378: Remove zarith (and take the one provided by dune-universe/opam-mirage-overlays)

@cdef629

Stages:

• ❌ x86_64-debian-10-4.13 (3m22s)
• 🟡 arm64-debian-10-4.13

History:

• 25/01/2022 15:42 ✔️ @4b04769 (18m42s)
• 25/01/2022 15:26 ❌ @cdef629 (3m22s)
• 25/01/2022 15:08 ❌ @43fab7a (9m52s)