Fix maven url

[koppor]

With Gradle 9.0. RC 1 I get

> Failed to apply plugin 'org.jabref.gradle.base.repositories'.
   > Cannot convert URI 'file:C:\git-repositories\JabRef/jablib/lib' to a file.

This PR fixes this.

Follow-up to #13319

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• [/] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [/] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[trag-bot]

@trag-bot didn't find any issues in the code! ✅✨