builder: invert missing arch support logic

The builder plugin checks that all of the requested architectures
are indeed supported, which is determined via the build tag. It
does that by constructing two sets, the requested architectures
and the supported architectures, and then constructing the set
of (asymmetric) differences between the one and the other. This
difference was meant to be "requested" - "supported", which then
will contain architectures that have been requested but are not
supported, or be empty in case all requested architectures are
indeed supported (the good case). However, previously the diff
was done the other way around ("supported" - "requested"), which
will then return a set of architectures that are supported but
were not requested, which is not what we want to check for.

Invert that the argument of the difference to indeed end up with
"requested" - "supported", which results in the correct check.

plugins/builder/osbuild.py

@@ -319,7 +319,7 @@ class OSBuildImage(BaseTaskHandler):
         # Architectures
         tag_arches = self.arches_for_config(buildconfig)
         arches = set(arches)
-        diff = tag_arches - arches
+        diff = arches - tag_arches
         if diff:
             raise koji.BuildError("Unsupported architecture(s): " + str(diff))