Disable csharp build on nightly macOS runs

[fbergmann]

Description

Nightly build and test fails on macOS on C# tests.

Motivation and Context

this disables the nightly build of macOS C# bindings while building them on win / linux. That way we have succeeding nightly builds again.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically