Add darwin/arm64 target once golang 1.16 is released

[yob]

Once golang 1.16 is released with support for m1 systems from Apple, it'd be neat to add it as a compilation target for this provider.

I haven't checked if terraform is officially shipping support for m1 system yet though, we should probably check that first.

[yob]

oh, goreleaser is already configured with goos=darwin and goarch=arm64. Maybe we'll get this for free in the next release now that we're using go 1.16. I'll leave this open until we see the next release and confirm there's a darwin/arm64 binary.