feat: Define src and target branch using vars

This will let you define the branch name for the PR you want to test
from the src repo and target branch to merge into, in the target repo

.gitlab-ci.yml

@@ -52,15 +52,15 @@ workflow:
   - cd CRI_XCBC
   - git config user.name "${GIT_AUTHOR_NAME}"
   - git config user.email "${GIT_AUTHOR_EMAIL}"
-  - git fetch origin uab-prod
-  - git fetch upstream dev
-  - git checkout uab-prod
-  - git merge origin/uab-prod
+  - git checkout ${EXT_PR_TARGET_BRANCH}
+  - git fetch origin ${EXT_PR_TARGET_BRANCH}
+  - git merge origin/${EXT_PR_TARGET_BRANCH}
   - git checkout -b integration
-  - git merge upstream/dev
   - export CRI_XCBC_HEAD=$(git rev-parse --short HEAD)
   - export CRI_XCBC_dev=$(git rev-parse --short upstream/dev)
   - export CRI_XCBC_prod=$(git rev-parse --short origin/uab-prod)
+  - git fetch upstream ${EXT_PR_SRC_BRANCH}
+  - git merge upstream/${EXT_PR_SRC_BRANCH}
   - cd ..
   - export PACKER_IMAGE_HEAD=$(git rev-parse --short HEAD)
   - echo CRI_XCBC_HEAD=${CRI_XCBC_HEAD} | tee -a $CI_PROJECT_DIR/image.env