ostbuild: Only check out source if we need to
ostbuild: Only check out source if we need to

When using --skip-built, we shouldn't do a checkout if we're not going
to build.

gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,resin-io/ostree,gatispaeglis/ostree,leandrosansilva/ostree,GNOME/ostree,sujitfulse/ostree,resin-io/ostree,gatispaeglis/ostree,leandrosansilva/ostree,resin-io/ostree,leandrosansilva/ostree,leandrosansilva/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,resin-io/ostree,GNOME/ostree,sujitfulse/ostree,sujitfulse/ostree,resin-io/ostree,leandrosansilva/ostree,gatispaeglis/ostree,sujitfulse/ostree,sujitfulse/ostree