fix target_stash

Signed-off-by: Oksana Salyk <oksana.salyk@hpe.com>

Author: osalyk

vars/unitTest.groovy

@@ -149,8 +149,7 @@ Map call(Map config = [:]) {
                  inst_repos: config.get('inst_repos', ''),
                  inst_rpms: inst_rpms)
     // image_version-compiler-build_type
-    String target_stash = image_version
-    target_stash += '-' + ${stage_info['compiler']}
+    String target_stash = "${image_version}-${stage_info['compiler']}"
     if (stage_info['build_type']) {
         target_stash += '-' + stage_info['build_type']
     }