Allow variables to be used in workflow:name

What does this MR do and why?

Describe in detail what your merge request does and why.

In !97502 (merged), we added workflow:name as an optional attribute, which applies a name to pipelines. This MR adds the ability to use variables in workflow:name. The variables can be nested.

#372538 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the pipeline_name feature flag by running the following in a Rails console: Feature.enable(:pipeline_name).
2. Add the following to .gitlab-ci.yml in GDK/GCK:

   workflow:
     name: '$PIPELINE_NAME'
     rules:
       - variables:
           PIPELINE_NAME: 'Pipeline for $CI_COMMIT_BRANCH'

3. Start a pipeline.
4. In rails console, check that the pipeline name is set to Pipeline for master (or whichever branch you ran the pipeline in): Ci::Pipeline.find(<id>).title

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.