for the inner elab.

	tb-set-elabinelab-cvstag dist-foo

Will result in this branch getting checked out from the mirrored repository
(updated nightly) on boss and sent back to the node, instead of the usual
source tarball that we keep in /usr/testbed/src (still the default
behaviour if no tag is specified. You can also do this if you like:

	tb-set-elabinelab-cvstag HEAD

which of course is a special tag to CVS.