ci: Fix date normalization in jenkins-failures.sh
ci: Fix date normalization in jenkins-failures.sh

We suggest that the user select a time via SINCE=`date -d -3days` but
that wasn't parsed correctly.

Signed-off-by: Ray Bejjani <7f525154e857f3d57a641a83dddc584f42addc6a@covalent.io>

tgraf/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,tklauser/cilium,cilium/cilium,scanf/cilium,scanf/cilium,tklauser/cilium,scanf/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,scanf/cilium,scanf/cilium,tklauser/cilium