builder: keep order of requested architectures

Preserve the order of architectures in the image requests inside
the compose requests, i.e. don't transform 'arches' into a set,
which has random order. It is not that anyone should really
depend on it, but there is also no need to mess with the order,
potentially making it harder for humans to match requested arches
with compose request content and logs.

plugins/builder/osbuild.py

@@ -318,8 +318,7 @@ class OSBuildImage(BaseTaskHandler):
 
         # Architectures
         tag_arches = self.arches_for_config(buildconfig)
-        arches = set(arches)
-        diff = arches - tag_arches
+        diff = set(arches) - tag_arches
         if diff:
             raise koji.BuildError("Unsupported architecture(s): " + str(diff))