The code was creating the cwd folder but when the workspace was
mounted in to the buildroot it was hiding the folder created in it
behind the bind mounted workspace.

However buy using the bubble warp `--dir` directive to ensure that cwd
exists then we can cover both workspace and non workspace situation with
the same method.

Build stream was only creating command-subdir for non workspace builds
this has now been fixed, and this regression test should support the
feature.