build: for linux/arm64 platform

[ab77]

Hi folks, hopefully it's as simple as that...

[palantirtech]

Thanks for your interest in palantir/policy-bot, @ab77! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks! I think this will do the right thing for the distribution, but I'll have to check if it will also build a linux/arm64 container, or if we need some extra configuration for that.

[bluekeyes]

After following up with internal teams, it looks like our build tool doesn't have full support for multi-architecture containers yet - I filed #558 to track fixing this. Until then, you may need to build your own container using the distribution (which should have both platform binaries once this merges.)