ci: compare using separate working copies

[merll]

📌 Summary

This PR makes minor improvements to the comparison check.

• Conflicts and other issues related to any file changes are avoided by running the test on two separate working copies, without changing branches.
• The main branch is no longer hard-coded, but taken from the PR.
• Additional manifest labels that invariably correspond with chart or sub-chart versions are removed. These may as well be reviewed in the file fill of the PR. This exclusion is optional in the script when using locally.

🔍 Reviewer Notes

🧹 Checklist

• Code is readable, maintainable, and robust.
• Unit tests added/updated