dcache-build: fix maven cache key-name

dcache-build.yml

@@ -12,7 +12,7 @@ variables:
 # Cache downloaded dependencies and plugins between builds.
 # To keep cache across branches add 'key: "$CI_JOB_NAME"'
 cache:
-  key: "CI_JOB_NAME"
+  key: "$CI_JOB_NAME"
   paths:
     - ./.m2/repository