Currently pkg-golang defaults to "." as a build target, generating a command like go build github.com/containerd/nerdctl/. However, the "/." is not valid, and the go command fails. There's no way to specify that we want to build just "github.com/containerd/nerdctl" with no sub-package (if the main.go is in the root).
It appears that now Golang supports build targets ending in "/." so this is resolved without action.