ci/b2c: rename IMAGE_UNDER_TEST to B2C_IMAGE_UNDER_TEST

Makes things mildly clearer, but more importantly gets rid of the
last variable used that doesn't start with `B2C_*` or `CI_*`.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33825>

.gitlab-ci/b2c/generate_b2c.py

@@ -40,7 +40,7 @@ values['ci_job_id'] = environ['CI_JOB_ID']
 values['ci_runner_description'] = environ['CI_RUNNER_DESCRIPTION']
 values['working_dir'] = environ['CI_PROJECT_DIR']
 
-values['image_under_test'] = environ['IMAGE_UNDER_TEST']
+values['image_under_test'] = environ['B2C_IMAGE_UNDER_TEST']
 values['machine_registration_image'] = environ['B2C_MACHINE_REGISTRATION_IMAGE']
 values['telegraf_image'] = environ['B2C_TELEGRAF_IMAGE']
 

.gitlab-ci/test/gitlab-ci.yml

@@ -366,8 +366,8 @@ yaml-toml-shell-py-test:
     # As noted in the top description, we make a distinction between the
     # container used by gitlab-runner to queue the work, and the container
     # used by the DUTs/test machines. To make this distinction quite clear,
-    # we rename the MESA_IMAGE variable into IMAGE_UNDER_TEST.
-    IMAGE_UNDER_TEST: "$MESA_IMAGE"
+    # we rename the variable.
+    B2C_IMAGE_UNDER_TEST: "$MESA_IMAGE"
 
     B2C_INSTALL_TARBALL_URL: "https://${PIPELINE_ARTIFACTS_BASE}/${S3_ARTIFACT_NAME}.tar.zst"
     B2C_CONTAINER_CMD: "./install/common/init-stage2.sh"