Fix path handeling for thermal ace generAtion

[janmalec]

Description

Description: This PR fixes a bug in openmc.data.ThermalScattering.from_njoy() where the ACE file is written to a temporary output_dir but the function attempts to read it from the current working directory, causing a FileNotFoundError.

Fixes #3170

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)