Enable markdownlint-cli2-formatter-pretty (by setting FORCE_COLOR=1)

Why is this change being made?

markdownlint-cli2-formatter-pretty doesn't do anything at the moment because it doesn't think the CI terminal (job log) supports color.

Setting this environment variable forces it to use color (as it is supported by the job logs)

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
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Commits

• Set FORCE_COLOR=1

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