Gitlab CI/CD: conditionally run downstream multistage pipeline from upstream pipeline - continuous-integration

Question

I am trying to trigger a downstream pipeline only when the artifact in the upstream pipeline is not available. I am using Gitlab CE v13.5.1.
The idea is:
Prebuild stage: curl from nexus and check if the artifact is
available. If yes, do not run the downstream trigger otherwise run
it.
to achieve this, when curl fails, I am creating a dummy exists.txt file which is then used in exists condition in later stages.
Run trigger only when exists.txt is present. But, problem is the trigger runs all the time irrespective of the exists.txt is present or not. So as to say, the exists condition in the trigger stage is not functioning.
Following is the gitlab-ci file:
image: ${NEXUS_REPO_DOCKER_URL}/<<image>>
variables:
CUSTOMER_NAME: "ABC"
CONFIG_VERSION: "1.2.7"
PICKER_TEMPLATE_TAG: "6.9.15"
stages:
- prebuild
- trigger
- build
- release
checkArtifactAvailability:
stage: prebuild
only:
- master
tags:
- docker
- lxc
script:
- set +e
- curl -s -S -f $NEXUS_RAW_PICKER_REPOSITORY/${PRODUCT_FLAVOR}/${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk --output ${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk
- "if [ '$?' -gt '0' ]; then touch exists.txt; fi"
-set -e
artifacts:
expire_in: 1 hour
paths:
- ${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk
- exists.txt
createApk:
stage: trigger
rules:
- exists:
- exists.txt
trigger:
project: dev/<<project_name>>
strategy: depend
when: on_failure
buildZip:
stage: build
tags:
- docker
- lxc
rules:
- exists:
- exists.txt
script:
- curl -s -S -f $NEXUS_RAW_PICKER_REPOSITORY/${PRODUCT_FLAVOR}/${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk --output ${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk
- zip -r v${CONFIG_VERSION}.zip <<files>> ${PRODUCT_FLAVOR}-${BUILD_TYPE}v${PICKER_TEMPLATE_TAG}.apk
artifacts:
expire_in: 1 hour
paths:
- v${CONFIG_VERSION}.zip
releaseToNexus:
stage: release
tags:
- docker
- lxc
rules:
- exists:
- exists.txt
needs:
- job: buildZip
artifacts: true
script:
- curl -u $NEXUS_USER:$NEXUS_PASSWORD --upload-file ./v${CONFIG_VERSION}.zip $NEXUS_RAW_QA_REPOSITORY/Picker/${CUSTOMER_NAME}/