CI: use a dedicated disk for releases (#12692)

This should make 'no space left on device' problems easier to handle

Use /tmp/releases as local_release_dir CI created machine, while keeping
the same folder on the runner (needed for gitlab-ci runner pods)

tests/scripts/testcases_run.sh

@@ -39,7 +39,6 @@ shift
 ansible-playbook \
     -e @tests/common_vars.yml \
     -e @tests/${TESTCASE_FILE} \
-    -e local_release_dir=${PWD}/downloads \
     "$@" \
     ${playbook}
 }