add darwin-arm64 for mac M1

[DannyGoldberg]

I added arm64 to build.sh so it would build for the new mac chips.

The build failed, so I ran the following, which created the go.mod and go.sum files:

go mod init main
go mod tidy

I then successfully ran build.sh to recreate the executables.

Finally, I tested aws-env-darwin-arm64 on my M1 Mac, which worked exactly as expected.