Add Arm64 on Darwin

[samjewell]

I've only done step 1 of Release Process: GoReleaser:
https://github.com/grafana/cortex-tools/blame/main/RELEASE.md#L6

For a couple of reasons:

1. I didn't want to create and push a new tag until this is approved & merged
2. I'm on an Arm64 Darwin machine myself, and so far I've found that when I try to build a Linux Amd64 Docker image, it never runs successfully on our linux machines on GCP - so I'm hoping someone on a linux machine (or an Amd64 Mac) can do step 3 for me (running goreleaser release).

I haven't tested the code change in this PR yet - I'm not sure how to exactly. I was looking in the Makefile for something I could run, but didn't spot anything yet. Cheers!

[samjewell]

Well the "build binaries: build all" step of the Github checks worked fine, and I was also able to run make all locally without any issues:

>$ file cortextool
cortextool: Mach-O 64-bit executable arm64
>$ ./cortextool version
version main

If I try make cross I get gox command not found. I thought go would natively compile for other platforms without any additional tooling 🤔

[gouthamve]

Running:

goreleaser build --rm-dist --skip-validate

Produced:

   • building binaries
      • building                  binary=/home/goutham/go/src/github.com/grafana/cortex-tools/dist/cortextool-darwin_darwin_amd64/cortextool
      • building                  binary=/home/goutham/go/src/github.com/grafana/cortex-tools/dist/cortextool-darwin_darwin_arm64/cortextool
      • building                  binary=/home/goutham/go/src/github.com/grafana/cortex-tools/dist/cortextool-linux_linux_amd64/cortextool
      • building                  binary=/home/goutham/go/src/github.com/grafana/cortex-tools/dist/cortextool-windows_windows_386/cortextool.exe
      • building                  binary=/home/goutham/go/src/github.com/grafana/cortex-tools/dist/cortextool-windows_windows_amd64/cortextool.exe

I think this is good enough, and I've asked Sam to verify if he can run the binary on an M1 Mac. Once I have confirmation, I'll merge this and cut the release.