Make bdist_* commands respect --skip-build passed to bdist (#10946).

There was already a test for this, but it was complicated and had a
subtle bug (custom command objects need to be put in dist.command_obj so
that other command objects may see them) that rendered it moot.
diff --git a/Lib/packaging/command/bdist_dumb.py b/Lib/packaging/command/bdist_dumb.py
index ed83c8e..d5773f0 100644
--- a/Lib/packaging/command/bdist_dumb.py
+++ b/Lib/packaging/command/bdist_dumb.py
@@ -55,7 +55,7 @@
         self.format = None
         self.keep_temp = False
         self.dist_dir = None
-        self.skip_build = False
+        self.skip_build = None
         self.relative = False
         self.owner = None
         self.group = None
@@ -73,7 +73,8 @@
                     "don't know how to create dumb built distributions "
                     "on platform %s" % os.name)
 
-        self.set_undefined_options('bdist', 'dist_dir', 'plat_name')
+        self.set_undefined_options('bdist',
+                                   'dist_dir', 'plat_name', 'skip_build')
 
     def run(self):
         if not self.skip_build: