Don't assume workdir "build" in CVS and Git steps (method 'copy')

Hi!

I just stumbled on this part where the CVS and Git source steps copy their checkout to the workdir…
Both steps assume the `workdir` is called 'build' in their `resetWorkdir` callbacks. Further, the CVS step already has a `srcdir` property in place which is not used anywhere.

This patch makes sure that the `copy` method keeps track of the actual previous `workdir` and the `resetWorkdir` callback actually resets it to the previous value.

The SVN step looks fine as it already seems to do something similar. Maybe someone with experience in the other VCSs should have a look at the other steps?