Propagate default workdir for newly created steps

The StepsYAMLParsingStep is statically in the step list of the factory,
so when starting a build Buildbot will call `setDefaultWorkdir` on each
step. We need to save this value and propagate it to the steps we
create on the fly to ensure they also receive a workdir value.

buildbot/master/files/config/factories.py

@@ -167,6 +167,7 @@ class StepsYAMLParsingStep(buildstep.ShellMixin, buildstep.BuildStep):
 
     haltOnFailure = True
     flunkOnFailure = True
+    workdir = None
 
     def __init__(self, builder_name, environment, yaml_path, **kwargs):
         kwargs = self.setupShellMixin(kwargs)
@@ -175,6 +176,10 @@ def __init__(self, builder_name, environment, yaml_path, **kwargs):
         self.environment = environment
         self.yaml_path = yaml_path
 
+    def setDefaultWorkdir(self, workdir):
+        buildstep.BuildStep.setDefaultWorkdir(self, workdir)
+        self.workdir = workdir
+
     @defer.inlineCallbacks
     def run(self):
         self.is_windows = re.match('windows.*', self.builder_name) is not None