chore(pipeline): allow to run pct tests on a limited plugin set

[lemeurherve]

This change allows to specify in a replay or via a commit a limited set of plugins on which to run pct tests, and marks the build as unstable if not empty to avoid any mistake or oversight.

Ref:

• Allow to run pct tests on a limited set of repositories and their plugin(s) #7216

Testing done

Replay with weekly-test label:

https://ci.jenkins.io/job/Tools/job/bom/view/change-requests/job/PR-7215/5/

-final String[] limitedPluginSet = []
+final String[] limitedPluginSet = [
+  'jenkinsci/pipeline-stage-view-plugin\tpipeline-rest-api,pipeline-stage-view',
+  'jenkinsci/ssh-agent-plugin\tssh-agent',
+]

Normal build with no value set:

https://ci.jenkins.io/job/Tools/job/bom/view/change-requests/job/PR-7215/6/stages/?selected-node=51

Build logs show that all repositories from the plugins.txt file generated by prep.sh are taken in account, as expected:

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[lemeurherve]

Using \t instead of in the example to avoid unexpected tab conversion into space on copy/paste.

A proper tab works too ofc.