Use YAML anchors and aliases to avoid repeating workflow conditions

[eddyashton]

It's rubbish that we keep repeating these complex conditions in every label-triggered job, and worse that they're slightly inaccurate (run on any trigger if label is present, rather than this label was added and also other trigger and label is present).

Found a blog from yesterday saying that we don't have to do the first part anymore!

https://github.blog/news-insights/product-news/lets-talk-about-github-actions/#h-yaml-anchors-reduce-duplication-in-complex-workflows
https://docs.github.com/en/actions/reference/workflows-and-actions/reusing-workflow-configurations#yaml-anchors-and-aliases

The magic words here for future reference are "YAML anchors and aliases", and we could use them all over the place to deduplicate. For now I'm just experimenting with replacing the long-test label condition.

[Copilot]

Pull request overview

This PR modernizes the GitHub Actions workflow by leveraging YAML anchors and aliases to eliminate duplication of conditional logic across multiple jobs. The change takes advantage of GitHub Actions' newly announced support for YAML anchors, replacing five identical conditional expressions with a single anchor definition and alias references.

• Defines a YAML anchor &check_trigger_conditions on the first job's condition
• Replaces four duplicate conditional expressions with alias references *check_trigger_conditions