S3-plugin cannot be noarch.

nbdkit.spec

@@ -472,12 +472,14 @@ This package lets you write Ruby plugins for %{name}.
 %endif
 
 
+# In theory this is noarch, but because plugins are placed in _libdir
+# which varies across architectures, RPM does not allow this.
 %package S3-plugin
 Summary:        Amazon S3 and Ceph plugin for %{name}
 License:        BSD
-BuildArch:      noarch
 
 Requires:       %{name}-python-plugin >= 1.22
+# XXX Should not need to add this.
 Requires:       python3-boto3