disable test run on forks

[AlexVCaron]

Quick description

Disable workflow run on forks. The workflow will still trigger, but all jobs will be skipped

Type of change

Check the relevant options.

• Bug fix (non-breaking change which fixes an issue)

Provide data, screenshots, command line to test (if relevant)

...

Checklist

• My code follows the style guidelines of this project (run autopep8)
• I added relevant citations to scripts, modules and functions docstrings and descriptions
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I moved all functions from the script file (except the argparser and main) to scilpy modules
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes