Currently there isn't a way to only build a content specification and then unzip it into a directory from the csprocessor directly. At the moment you would either have to run "csprocessor assemble" or "csprocessor build" and then an unzip command.
Alternative: Perhaps have build not create the zip at all and just create the directory structure with files and then have a "--zip" option (or similar) to produce a ZIP output instead. This alternative is probably better as most users will want just the inner content and not care about sending the ZIP. However adding the "--zip" option will still allow you to create a build to send if required.
Does csprocessor assemble --no-build do this?
No, that will run publican as well, assuming the ZIP exists. If the ZIP doesn't exist than you'd just get an error. FWIW, "--no-build" will not run "csprocessor build", so it's use case is when you have received the ZIP and then want to assemble/preview it yourself.