rbuilder: Support darwin/arm64

[alexanderbez]

Add support for the new darwin/arm64 architecture to rbuilder's reproducible builds.

/cc @alessio @marbar3778

[tac0turtle]

Hey, thanks for opening an issue. This feature would be awesome. Would you like to open a PR?

[alessio]

I suspect that this is supported already. Take example from https://github.com/tendermint/images/tree/master/rbuilder#makefile-integration, and try to pass --env TARGET_PLATFORMS='darwin/arm64' to docker run. Please report here any findings @alexanderbez!