Gradle: Fix dependency cache

As part of #135 and this last 1dda7bd
commit I incorrectly removed the 'DEP_CACHE_FOLDER_NAME' variable.
Without this variable set, placing Gradle dependency cache is not
working as expected due to the fact that this 'DEP_CACHE_FOLDER_NAME'
variable is now unbound. And this makes this whole dependency cache
solution not working at all.

bin/restore_gradle_dependency_cache

@@ -9,6 +9,7 @@ echo "Restoring Gradle dependency cache..."
 GRADLE_DEP_CACHE="$GRADLE_HOME/dependency-cache"
 
 DEP_CACHE_BASE_FOLDER=$(dirname "$GRADLE_DEP_CACHE")
+DEP_CACHE_FOLDER_NAME=$(basename "$GRADLE_DEP_CACHE")
 
 # `save_cache` & `restore_cache` scripts only work if they are called from the same directory
 pushd "$DEP_CACHE_BASE_FOLDER"