[build] pass package name and variant to build

The targets which are invoking pm_build_package directly (update.meta and
system_image.meta) are producing packages with incorrect names. This used to
be ignored due to bugs in the publish path that hid the problem, but those
were recently fixed highlighting this bug.

Test: fx build, observe "update" instead of "update.meta" package produced.
Change-Id: I947fa915e9ad7f35561ed463e03d79b61da2ebe0