Combine mock stderr and stdout logs

Mock doesn't normally log anything to stdout - so it's confusing to mention
separate logs in the messages. Combine the two output streams together.
(This is what koji does as well.)

module_build_service/builder/KojiModuleBuilder.py

@@ -356,7 +356,7 @@ class KojiModuleBuilder(GenericBuilder):
         fd.close()
         log.debug("Building %s.spec" % name)
 
-        # We are not interested in the rpmbuild stdout...
+        # We are not interested in the rpmbuild output...
         null_fd = open(os.devnull, "w")
         execute_cmd(
             [
@@ -369,7 +369,7 @@ class KojiModuleBuilder(GenericBuilder):
                 "_sourcedir %s" % sources_dir,
             ],
             cwd=td,
-            stdout=null_fd,
+            output=null_fd,
         )
         null_fd.close()
         sdir = os.path.join(td, "SRPMS")