packit: use post-upstream-clone to create spec
packit: use post-upstream-clone to create spec

The latest sandbox container version requires the spec file to already
exist at the start of the `create-archive` action.  Move our
substitutions to `post-upstream-clone`.

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