packit.yaml: use webpack-jumpstart --rebase
packit.yaml: use webpack-jumpstart --rebase

Get packit to build the exact same thing that GitHub did.

We move the download to before the spec file generation, since the extra
file in the tree prevents us from rebasing.

martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit