fix: handle date format for e2e cleanup

[mweibel]

fixes issue with cleanup reported here: https://github.com/cloudscale-ch/cluster-api-provider-cloudscale/actions/runs/24622224615/job/71994951165

and fixes a shellcheck issue:

In hack/clean-e2e-images.sh line 24:
cutoff=$(date -u -v-${MAX_AGE_DAYS}d +%s 2>/dev/null || date -u -d "${MAX_AGE_DAYS} days ago" +%s)
                    ^-------------^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean:
cutoff=$(date -u -v-"${MAX_AGE_DAYS}"d +%s 2>/dev/null || date -u -d "${MAX_AGE_DAYS} days ago" +%s)

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ..