This makes it easier to handle this value in formula and pass it to
other directives (like a bash directive), or modify it (append something
to it).
This is a substantial change. None of existing tasks seem to be making
use of multi-valued ${arch}. Generic tasks usually use 'all' arch value
in koji directive and process everything in one pass. Arch-specific
tasks will mostly want to handle just a single arch in a single pass.
That will also help us keep our sanity.
This patch is helpful for resolving T894 (making depcheck/rpmdeplint a
per-arch task).