There is also a macos/arm64 binary of opam available from the releases page for your convenience, and opam repository has been updated to understand the new tier-1 constraints imposed by macos/arm (i.e. the only working compilers there are 4.10.2 and 4.12.0~dev, and opam init will now do the right thing).
There will be a number of packages that are broken due to the shift to /opt/homebrew from /usr/local for Homebrew/ARM (due to the need to keep them simultaneously installed on the same Mac), so please feel free to submit PRs to opam-repository to fix this stuff.
We’ll shortly have Mac (both Intel and ARM) testing up and running on opam-repository, so CI will catch up with reality once more, thanks to furious hacking by @patricoferris to extend our ocurrent-based CI infrastructure to support the unique vagaries of the Mac environment (notably, a total lack of native containers). We have it working locally, and are just upstreaming it now.