This addresses the surprising behavior that the StoreBoxMetadata hook
was running many times during a machine up, including during failed
operations where a destroy_on_error deleted the machine. This was
resulting in an error that looked like:
> No such file or directory @ rb_sysopen [...] /[...]/box_meta
Plugin action hooks using prepend/append were attaching every time a
Builder was run, including sub-Builders that show up for things like
Call actions.
To fix this, we tell Builders if they are "primary" and only run
prepend/append on those. See inline comments for more explanation.
