fix: use configured cache path in getDiffStats instead of hardcoded default

[Fikri-20]

getDiffStats() was resolving the .git directory against Settings.DEFAULT_CACHE_PATH instead of config.getCachePath(). So running with --cache-path /tmp/my-cache would clone the plugin correctly there but then fail to read it back when building the PR description. One-line fix.

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Testing Done

I tested it locally, and here is the before and after

Before (main — GHServiceTest):

[INFO] Running io.jenkins.tools.pluginmodernizer.core.github.GHServiceTest
[INFO] Tests run: 48, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 15.46 s

After (fix/get-diff-stats-cache-path — mvn clean test -pl plugin-modernizer-core):

[INFO] Tests run: 432, Failures: 0, Errors: 0, Skipped: 1
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  02:55 min
[INFO] Finished at: 2026-04-17T09:11:14+02:00

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 demonstrates feature works or fixes the issue