Exclude readonly pipelines from Chain::Skip to prevent ActiveRecord::ReadOnlyRecord error

What does this MR do and why?

Adds a condition to Chain::Skip for readonly pipelines, which is causing an ActiveRecord::ReadOnlyRecord error.

This happens because during this part of the chain, if we have a pipeline.readonly, then calling pipeline.skip on it will raise this error.

The easiest way to recreate this is to do it in the CI editor and compare behaviors in main/master and this brach:

1. Add the following script to a simple .gitlab-ci.yml:

job:
  script: ls
  rules:
    - if: $CI_PIPELINE_SOURCE == "schedule"
      when: never
    - when: on_success

2. Check the network tab and you will see a failing graphql request with the error - Ci::Pipeline is marked as readonly

The linter will also either load infinitely or show an error: (it should probably not load infinitely but that's a FE problem for now):

which is because of ActiveRecord::ReadOnlyRecord error.

You can further verify this in the console:

pipeline = ::Ci::Pipeline.first # any pipeline will do
pipeline.readonly! # Make it readonly if it's not already
pipeline.skip # this is what we are trying to avoid in the MR

you will get the error: ActiveRecord::ReadOnlyRecord: Ci::Pipeline is marked as readonly

References

#428590 (closed)

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