FlubuCore dotnet cli tool not working on .Net Core 3.1

It requires that .Net Core sdk 2.X is installed.

As Alternative FlubuCore global tool can be used instead.

```
Run dotnet flubu compile.linux
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
- The following frameworks were found:
3.1.1 at [/opt/hostedtoolcache/dncs/3.1.101/x64/shared/Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
- https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=2.1.0&arch=x64&rid=ubuntu.18.04-x64
```