build-qemu-robot-docker.sh: update to plucky

The previous version has gone EOL

Change-Id: Ic9522129e17a6359a49b7720f10a7b5229163c3e
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/scripts/build-qemu-robot-docker.sh b/scripts/build-qemu-robot-docker.sh
index 42adb2f..e251d88 100755
--- a/scripts/build-qemu-robot-docker.sh
+++ b/scripts/build-qemu-robot-docker.sh
@@ -21,7 +21,7 @@
 http_proxy=${http_proxy:-}
 
 DOCKER_IMG_NAME=${1:-"openbmc/ubuntu-robot-qemu"}
-DISTRO=${2:-"ubuntu:oracular"}
+DISTRO=${2:-"ubuntu:plucky"}
 UBUNTU_MIRROR=${UBUNTU_MIRROR:-""}
 PIP_MIRROR=${PIP_MIRROR:-""}
 docker_reg=${DOCKER_REG:-"public.ecr.aws/ubuntu"}