element.py: Preserve workspace state in failed build sandboxes.

As reported in regression #346, since we started mounting the workspaces
we now fail to debug build failures for workspaces, the shell command
no longer sees the workspaces files to debug.

As the failed sysroot represents a very particular build failure, it
is incorrect to attempt to remount a workspace again after a failed
build, as the workspace files may have changed since the build.

Instead, in the case of a build failure on a workspaced element, we
copy the files from the workspace in place (while automatically preserving
the mtimes and everything, as utils.safe_copy() does).

When we implement failed build artifacts, this case will have to be
handled in a very similar way, too.

This fixes issue #346