Add --discard flag to build command

[kofalt]

Right now, if you're experimenting with a build you can run: hroot build -d file:///dev/null, which will avoid saving an unwanted change to your graph.

But this is not optimal: it still serializes the tar (which is immediately lost). Two changes:

• Add a --discard flag that does not attempt to export.
• If destination file:///dev/null, equivalent to --discard