Add build target/target-file flags to build subcommand

[Shados]

Split off from #35.

Adds:

• A buildTargets argument to the Nix machine builder expression, that expects an attribute set of the form { outputLinkName = node: node.config.some.attribute.to.build.per.machine; } and replaces the default build targets based on this, if given

• A --target flag on morph build that allows specifying a single buildTargets set member on the CLI as a lambda, e.g. morph build some/deployment.nix --target 'node: node.config.environment.etc."nix/nix.conf"'

• A --target-file flag on morph build that makes it convenient to specify a file to import containing a valid buildTargets expression, e.g. morph build some/deployment.nix --build-targets='./manual-epub.nix'

  Where the contents of manual-epub.nix are:

  {
    "manual" = node: node.config.system.build.manual.manualEpub;
  }