add arm64 to goreleaser builds

[oliverbr]

Hi and thank you very much for this project!

We are currently migrating some of our instances to ARM-based builds and calert is one of the few things keeping us from doing so for our monitoring stack.

I have only tested building locally on macOS with goreleaser build --snapshot so far, but currently lack the resources to test this any further. I am, however, fairly confident that this should "just work" (famous last words, I know).

I also didn't extend the dockers section because I didn't want to introduce a tag naming scheme, etc.

[mr-karan]

I am, however, fairly confident that this should "just work" (famous last words, I know).

:smile:

Yeah, I think this should be fine.

If it's okay, I'm thinking to address https://github.com/mr-karan/calert/issues/44 before cutting a new release.

[oliverbr]

Absolutely fine by me ;)