testing/pdal: Add comment on why x86_64 only

Added a comment to explain why he package is only provided for x86_64: It's
dependencies are not provided for the other architectures.
7 jobs for !561 with pdal in 4 minutes and 21 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #5861
docker-alpine x86_64
lint

00:00:18

 
  Build
passed #5866
docker-alpine ci-build aarch64
build-aarch64

00:00:20

passed #5867
docker-alpine ci-build armv7
build-armv7

00:00:21

passed #5865
docker-alpine ci-build ppc64le
build-ppc64le

00:00:25

passed #5864
docker-alpine ci-build s390x
build-s390x

00:00:19

passed #5863
docker-alpine ci-build x86
build-x86

00:00:21

passed #5862
docker-alpine ci-build x86_64
build-x86_64

00:03:36