[Bug] Custom source generator in solution

When generating documentation for a solution that has a source generator as project which is used by other projects in the same solution the build fails as the source generator never runs.

**To Reproduce**
Steps to reproduce the behavior:
1. Add a custom source generator to your solution
2. Make another project in the solution use it

**Expected behavior**
Should have the same behaviour as dotnet build does.

**Context (please complete the following information):**
- OS: Linux ( Fedora 42 )
- Docfx version: 2.78.3+850a61c2ecb83b0bda21448f1393e480934556fd