NUnit.Console3.12-beta1 support netcore

In [NUnit.Console3.12-beta1](https://github.com/nunit/nunit-console/releases/tag/v3.12.0-beta1) added net core and linux support but Cake can't run new version of tool, when tool path set to .dll instead .exe.

### What You Are Seeing?
When i set tool path to `nunit3-console.dll`, i receive `NUnit3: Unrecognised error (exit code -532462766)`.

### What is Expected?
Tool is running using `dotnet nunit3-console.dll path/to/tests/tests.dll`

### What version of Cake are you using?
0.38.5

### Are you running on a 32 or 64 bit system?
64bit

### What environment are you running on? Windows? Linux? Mac?
Windows

### Are you running on a CI Server? If so, which one?
No

Start by tracing Cake's NUnit console tool invocation and reproduce the reported Windows case with the tool path set to nunit3-console.dll. Verify how executable paths are handled and confirm that the tool runs through dotnet with the test assembly; done means the reported NUnit3 error is resolved for the beta version.

Written by the indexing model from the issue text.