Fix JabRef-post-image.wsf to make it agnostic to the location of the …

[alexeysemenyukoracle]

Fix JabRef-post-image.wsf script to make it independent of the project build location.

Build-only change to eliminate dependencies on the context in which the "post-image" jpackage script is called on Windows.

Closes #13034

Important: the baseline for the fix is v6.0-alpha2 tag. There is no JabRef-post-image.wsf in the main branch after https://github.com/JabRef/jabref/pull/12990, so this PR can not be integrated as-is.

Mandatory checks

• [x] 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.