Fixed the clean command to use --target since -t isn't always available. (#2378)

Author: bvanessen

.gitlab/common/common.yml

@@ -118,7 +118,8 @@
   script:
     - echo "== CLEAN BUILD DIRECTORY =="
     - source spack-ci-env-name.sh
-    - cmake --build ${LBANN_BUILD_PARENT_DIR}/build -t clean
+    - echo "== CLEANING ${LBANN_BUILD_PARENT_DIR}/build =="
+    - cmake --build ${LBANN_BUILD_PARENT_DIR}/build --target clean
     - echo "== REMOVING BUILD RESOURCES =="
     - find ${CI_PROJECT_DIR}/builds -maxdepth 1 -mtime +2 -name "lbann_*" -type d -print -exec rm -r {} \;
     - find ${CI_PROJECT_DIR} -maxdepth 1 -mtime +2 -name "LBANN_*_setup_build_tools.sh" -type f -print -exec rm {} \;