main/etckeeper: upgrade to 1.18.15

upstream is no longer shipping the cron job
7 jobs for !14915 with etckeeper-1.18.15 in 1 minute and 22 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #253821
docker-alpine x86_64
lint

00:00:25

 
  Build
passed #253826
docker-alpine ci-build aarch64
build-aarch64

00:00:33

passed #253827
docker-alpine ci-build armv7
build-armv7

00:00:39

passed #253825
docker-alpine ci-build ppc64le
build-ppc64le

00:00:55

passed #253824
docker-alpine ci-build s390x
build-s390x

00:00:51

passed #253823
docker-alpine ci-build x86
build-x86

00:00:27

passed #253822
docker-alpine ci-build x86_64
build-x86_64

00:00:30