Hi,
I think GitHub doesn't yet host any M1-based Mac OS X runners, though they
have added for self-hosted M1 runners. Cross-building from x86 Mac OS X would
probably be possible, but it wouldn't allow automatic testing of the built packages.
So I'll consider adding M1 packages when GitHub adds the support:
https://github.com/github/roadmap/issues/528
While it is not quite same, I'd suggest using Python's pip to install nanopb.
It can be done inside a virtualenv and it brings in protoc also (via grpcio-tools package).
--
Petteri