Fix CA1839: Use 'Environment.ProcessPath'

cc: @iSazonov

Fix [CA1839: Use 'Environment.ProcessPath'](https://docs.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1839)

```
/home/vsts/work/1/s/test/xUnit/csharp/test_FileSystemProvider.cs(118,49): error CA1839: Use 'Environment.ProcessPath' instead of 'Process.GetCurrentProcess().MainModule.FileName' [/home/vsts/work/1/s/test/xUnit/xUnit.tests.csproj]
```