Remove gox in favor of go build.
`gox` hasn't had a release to update it in many years, so is missing
support for many modern systems, like `darwin/arm64`.
In any case, we only use it for dev builds, where we don't even use
the ability of it to build for multiple platforms. Release builds use
`go build` now.
So, this switches to `go build` everywhere.
I pulled this down and tested it in Windows as well. (Side note: I
couldn't get `gox` to work in Windows, so couldn't build before this
change.)
