HDDS-15675. Ranger test overrides custom OZONE_RUNNER_IMAGE

[adoroszlai]

What changes were proposed in this pull request?

.env files define default values for variables in Docker Compose. test-ranger.sh evaluates .env as a whole to get RANGER_VERSION, which it needs for downloading Ranger. This overrides any custom value for other variables that appear in .env and are already defined (e.g. OZONE_RUNNER_IMAGE).

https://issues.apache.org/jira/browse/HDDS-15675

How was this patch tested?

Added set -x and early exit, then ran test-ranger.sh without/with RANGER_VERSION being defined:

$ hadoop-ozone/dist/target/ozone-2.3.0-SNAPSHOT/compose/ozonesecure-ha/test-ranger.sh   
+ [[ -z '' ]]
+ export RANGER_VERSION=2.8.0
+ RANGER_VERSION=2.8.0
+ exit

$ RANGER_VERSION=2.9.0 hadoop-ozone/dist/target/ozone-2.3.0-SNAPSHOT/compose/ozonesecure-ha/test-ranger.sh   
+ [[ -z 2.9.0 ]]
+ exit

CI:
https://github.com/adoroszlai/ozone/actions/runs/28175310084/job/83452870581

[dombizita]

Thanks for fixing this @adoroszlai, the change makes sense!

[adoroszlai]

Thanks @dombizita for the review.