Skip building multi-arch binary-less image

This commit allows IIB to process a single build for binaryless images
with a generic architecture labeled as "linux-amd64"

Refers to CLOUDDST-23383

iib/workers/tasks/utils.py

@@ -1165,12 +1165,6 @@ def prepare_request_for_build(
         build_request_config,
         [("from_index", "v4.5"), ("source_from_index", "v4.5"), ("target_index", "v4.6")],
     )
-    arches = gather_index_image_arches(build_request_config, index_info)
-    if not arches:
-        raise IIBError('No arches were provided to build the index image')
-
-    arches_str = ', '.join(sorted(arches))
-    log.debug('Set to build the index image for the following arches: %s', arches_str)
 
     # Use the distribution_scope of the from_index as the resolved distribution scope for `Add`,
     # and 'Rm' requests, but use the distribution_scope of the target_index as the resolved
@@ -1191,9 +1185,11 @@ def prepare_request_for_build(
 
     binary_image = build_request_config.binary_image(request_index, distribution_scope)
     if binary_image == "scratch":  # binaryless image mode
+        arches = set(["amd64"])
         binary_image_resolved = binary_image
         binary_image_arches = arches
     else:
+        arches = gather_index_image_arches(build_request_config, index_info)
         binary_image_resolved = get_resolved_image(binary_image)
         binary_image_arches = get_image_arches(binary_image_resolved)
 
@@ -1204,6 +1200,9 @@ def prepare_request_for_build(
             )
         )
 
+    arches_str = ', '.join(sorted(arches))
+    log.debug('Set to build the index image for the following arches: %s', arches_str)
+
     bundle_mapping: Dict[str, Any] = {}
     for bundle in bundles:
         operator = get_image_label(bundle, 'operators.operatorframework.io.bundle.package.v1')

tests/test_workers/test_tasks/test_utils.py

@@ -1116,7 +1116,7 @@ def test_prepare_request_for_build_binaryless(
             _binary_image=binary_image,
             from_index=from_index,
             overwrite_from_index_token=None,
-            add_arches=["amd64"],
+            add_arches=["s390x"],
             bundles=bundles,
             distribution_scope="prod",
             binary_image_config=binary_image_config,