MT#61101 ec2-create-ce: fix disabling backports repository

Followup fix for commit 5e6c5363a1

grep interprets the provided argument `-backports` as option. We need to
mark the end of the grep options accordingly via "--", to avoid failure
during grep execution.

Change-Id: I6f0360a34583b2d0e961d282be16b3f6e90445a2

ec2-create-ce

@@ -454,7 +454,7 @@ if [ -f /etc/apt/sources.list.d/backports.list ] && grep -q 'jessie-backports' /
 fi
 
 # workaround for mr13.2.1 and newer with bookworm-backports enabled by default:
-if [ -f /etc/apt/sources.list.d/debian.sources ] && grep -q '-backports' /etc/apt/sources.list.d/debian.sources ; then
+if [ -f /etc/apt/sources.list.d/debian.sources ] && grep -q -- '-backports' /etc/apt/sources.list.d/debian.sources ; then
   echo "backports is unexpected, disabling from /etc/apt/sources.list.d/debian.sources"
   sed -i -E 's/ [a-z]+-backports//' /etc/apt/sources.list.d/debian.sources
 fi