just: Use gh if available

[koppor]

When trying out a PR, I sometimes don't want to type two commands.

gg.cmd just run-pr xyz

is enough for me.

I know we have JBang scripts for PR checkout, but I would like to use gh when possible.

Thus, this change

Steps to test

1. sh gg.cmd just run-pr 14486 and see JabRef Gui starting

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[koppor]

I just merge - if something is not working with users following https://blog.jabref.org/2025/05/31/run-pr/, we can revert.