cmd-import: consider architecture when calculating previous build

Not all architectures get built for every build in a stream. This is
true at least for non-production streams. Let's account for that here.

Author: dustymabe

src/cmd-import

@@ -39,10 +39,12 @@ def main():
 
     builds = Builds()
     arch = get_basearch()
+    previous_build = builds.get_latest_for_arch(arch)
     if builds.has(buildid) and arch in builds.get_build_arches(buildid):
         print(f"ERROR: Build ID {buildid} ({arch}) already exists!")
         sys.exit(1)
 
+
     with tempfile.TemporaryDirectory(prefix='cosa-import-', dir='tmp') as tmpd:
         # create the OCI archive
         tmp_oci_archive = generate_oci_archive(args, tmpd)
@@ -68,8 +70,8 @@ def main():
     # right places) let's use `cosa diff` to insert package and
     # advisory diffs into the meta.json and log the RPM diff, but
     # only if we have something to diff against.
-    if args.parent_build or len(builds.get_builds()) > 1:
-        starting_buildid = args.parent_build if args.parent_build else builds.get_previous()
+    if args.parent_build or previous_build:
+        starting_buildid = args.parent_build if args.parent_build else previous_build
         calculate_and_log_diffs(builds, buildid, arch, starting_buildid)
 
     if not args.skip_prune: