Store date in variable instead of recomputing it dynamically

[fwcd]

If the date changes during a CI build (e.g. because the workflow starts before midnight and ends after), it will fail as evidenced e.g. by https://github.com/fwcd/archlinuxarm-images/actions/runs/6017874327/job/16324990053.

To fix this, we should output the date once in a stage and then reference it instead of recomputing it on-the-fly.