Update .gitlab-ci.yml file to use CI cache to improve CI performance

- referenced: https://qiita.com/SatoshiTerasaki/items/4be31630955b3dbe3bbc#cache-に関して

.gitlab-ci.yml

@@ -1,3 +1,14 @@
+variables:
+  CI_JULIA_CACHE_DIR: ${CI_PROJECT_DIR}/julia_pkg
+  JULIA_DEPOT_PATH: ${CI_JULIA_CACHE_DIR}
+cache:
+  key:
+    files:
+      - Project.toml
+      - docs/Project.toml
+    prefix: ${CI_JOB_NAME}
+  paths:
+    - ${CI_JULIA_CACHE_DIR}
 .script:
   script:
     - |