Cache vendor directory and install dependencies also when building phar.

.gitlab-ci.yml

 image: composer:2
 
+cache:
+    key: ${CI_COMMIT_REF_SLUG}
+    paths:
+        - vendor/
+
+before_script:
+    - composer install
+
 test:
     script:
         - composer install --ignore-platform-reqs
         - composer run php-cs:check
-#        - vendor/bin/phpunit
 
 build:
     script: