Ensure that Model.run() works when specifying a custom XML path

[paulromano]

Description

The Model.run() method allows you to specify keyword arguments that are passed to export_to_model_xml. If you pass a path keyword argument, it will be used to write the corresponding XML file but it is not taken into account in the call to openmc.run which will cause it to look for "model.xml" (or a set of individual XML files). This change ensures that when the user calls model.run(path=...), the right path is used in the call to openmc.run().

Fixes # (issue)

Checklist

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