multi-arch build process / release improvements

Go build step requires cgo, so the release process is aided by https://github.com/cbroglie/goreleaser-cgo, which is a thin wrapper around https://github.com/docker/golang-cross. (Introduced in https://github.com/cloudflare/cfssl/pull/1048) However, it doesn't support go 1.14 or 1.15 currently: https://github.com/docker/golang-cross/issues/64.
One alternative could be https://github.com/elastic/golang-crossbuild. It would also make future PRs easier if the goreleaser-cgo image was part of the repo.

Also https://pkg.cfssl.org/ is trailing behind by a couple versions, I'm going to link our internal ticket to this issue.