downloadunpacked: Because shared part should not be uninstalled, install...

downloadunpacked: Because shared part should not be uninstalled, install method should return an empty list in shared mode.

slapos/recipe/downloadunpacked.py

@@ -161,6 +161,7 @@ class Recipe:
     if self.shared:
       with open(os.path.join(self.parts, ".slapos.recipe.build.signature"), 'w') as f:
         f.write('\n'.join(self._debug_signature_text))
+      return []
     if self.parts is not None:
       return [self.parts]
     else: