Fix update-tests-metadata CI step failing in master

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30627 changed
`JSON.parse` to `Gitlab::JSON.parse`, but the latter isn't available in
the simple CI script since it doesn't load a Rails environment.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/216349

lib/rspec_flaky/report.rb

@@ -19,7 +19,7 @@ module RspecFlaky
     end
 
     def self.load_json(json)
-      new(RspecFlaky::FlakyExamplesCollection.new(Gitlab::Json.parse(json)))
+      new(RspecFlaky::FlakyExamplesCollection.new(JSON.parse(json)))
     end
 
     def initialize(flaky_examples)