gitlab-ci: Install grep instead of using busybox version

.gitlab-ci.yml

@@ -19,6 +19,8 @@ check:
 codeCoverage:
     stage: test
     coverage: '/Total.*?([0-9]{1,3})%/'
+    before_script:
+        - apk add --no-cache grep
     script:
         - ./gradlew codeCoverageReport
         # The actual output that will be parsed by the code coverage