Disable Ubuntu arm32 corefx xunit test result parsing

Disable Ubuntu arm32 corefx xunit test result parsing

We have a problem with the xunit plug-in, where it is consistently
failing on Ubuntu arm32 test result uploading with this error:

```
The plugin hasn't been performed correctly
```

We haven't been able to identify the reason. So, do not add xunit parsing of the test data in this scenario.
This is tracked by: https://github.com/dotnet/coreclr/issues/19447.

Commit migrated from https://github.com/dotnet/coreclr/commit/da65d2f15084c45b8a352ebe901de9cbdba7add0