Replace obsolete CI variable.

.gitlab-ci.yml

@@ -10,7 +10,7 @@ stages:
 - check
 
 variables:
-  TEST_IMAGE: briar/reproducer:${CI_BUILD_REF_NAME}
+  TEST_IMAGE: briar/reproducer:${CI_COMMIT_REF_NAME}
   RELEASE_IMAGE: briar/reproducer:latest
   TAG_IMAGE: briar/reproducer:${APP}-${RELEASE_TAG}