ci: Fix mkpipeline trimming

[def-]

Seen in https://buildkite.com/materialize/test/builds/116699

Follow-up to #35014

Test run, should trim everything and does: https://buildkite.com/materialize/test/builds/116706
Test run, should trim nothing and does: https://buildkite.com/materialize/test/builds/116707

[github-actions]

Thanks for opening this PR! Here are a few tips to help make the review process smooth for everyone.

PR title guidelines

• Use imperative mood: "Fix X" not "Fixed X" or "Fixes X"
• Be specific: "Fix panic in catalog sync when controller restarts" not "Fix bug" or "Update catalog code"
• Prefix with area if helpful: compute: , storage: , adapter: , sql:

Pre-merge checklist

• The PR title is descriptive and will make sense in the git log.
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[ggevay]

Thank you!