fix(alchemy_link.plugin): Avoid getting an absolute URL

[tvdeyen]

What is this pull request for?

Suddenly the anchor.href attribute returns an absolute URL - at least in Firefox - with the Alchemy Admin URL as base.

We want a relative URL.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.94%. Comparing base (a1e0391) to head (af84b09).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2885 +/- ## ======================================= Coverage 95.94% 95.94% ======================================= Files 230 230 Lines 6260 6260 ======================================= Hits 6006 6006 Misses 254 254 ```

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