Support build args without explicit value

[alebcay]

This PR fixes https://github.com/containerd/nerdctl/issues/1408.

When a build arg key is specified without an explicit value, the value should be taken from the corresponding environment variable on the host.

When the corresponding environment variable on the host is unset, the build arg should be ignored to avoid masking a possible default value specified in the Dockerfile. To pass an empty string as build arg, use the form VAR= on the CLI/as an environment variable.