This in-progress feature will run a dynamically generated set of
buildFinished hooks, which must be nested under the `runCommandHook.*`
attribute set. This implementation is not very good, with some to-dos:
1. Only run if the build succeeded
2. Verify the output is named $out and that it is an executable file
(or a symlink to a file)
3. Require the jobset itself have a flag enabling the feature, since
this feature can be a bit dangerous if various people of different
trust levels can create the jobs.
e56c49333f