Set GRADLE_USER_HOME on CI via variables instead of before_script

.gitlab-ci.yml

@@ -5,9 +5,7 @@ stages:
 
 variables:
   GRADLE_ARGS: --no-daemon --stacktrace --warning-mode=all
-
-before_script:
-  - export GRADLE_USER_HOME=`pwd`/.gradle
+  GRADLE_USER_HOME: "$CI_PROJECT_DIR/.gradle"
 
 tests:
   stage: test