[ci] fix folder check command

.gitlab-ci/default.yml

@@ -13,7 +13,7 @@ workflow:
 # variables that may be overwritten by the trigger
 variables:
   DUMUX_REPO: "https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git"
-  DUMUX_FOLDER: "dumux"
+  DUMUX_FOLDER: "DUMUX"
   MR_TARGET_BRANCH_NAME: ""
   DUMUX_MR_SOURCE_BRANCH: ""
   DUMUX_MR_TARGET_BRANCH: ""
@@ -24,7 +24,7 @@ select tests:
   stage: configure
   before_script:
     - |
-      if [ ! -d $DUMUX_FOLDER ]; then
+      if [ ! -d "$DUMUX_FOLDER" ]; then
           git clone -b master --depth 1 $DUMUX_REPO $DUMUX_FOLDER
       fi