Recent refactoring replaced the CreateWorkspace function call with a
dedicated XML step (for clarity, and because it avoids needless
recreation of workspaces). In the proxy function, the same
CreateWorkspace xml step has been added, cloning the OAI RAN repo, when
we actually need the repository for the proxy. Hence, fix this by
calling the CreateWorkspace() function manually, but only when
necessary, to build the proxy. (It would only be a problem if the
pipeline would rebuild the proxy, which did not happen because not much
work is happening on the proxy repo).

Further, refactoring and simplication of the proxy build function were
done:

- never tag proxy with develop, that makes no sense
- overwrite CreateWorkspace() info after having created and used the
  workspace (the "if" to build the proxy)
- simplify copying of logs
- add simple unit test
- remove check for ubuntu; the dependcy is "docker". Simplify the code
  to not obscure with a class variable.