nBuildKit needs .net core 3.0+ runtime to be installed if generating test reports

[JosephekcoL]

In feature/netstandardversions, the version of ReportGenerator was updated from 2.4.5 to 4.1.9

The new version only has executables for .net47 and .netcore3.0.

Because the SearchPackagesDirectoryForToolPath task will pick the last (highest in this case) executable, the .netcore3.0 executable is used and the build will fail with the following error:

build.test.unit.vstest.msbuild(389,9): error NBK0502: It was not possible to find any compatible Framework version
build.test.unit.vstest.msbuild(389,9): error NBK0502: The specified framework 'Microsoft.NETCore.App', version '3.0.0' was not found.

This isn't a big issue in our case because it will run fine with .netcore3.1 installed but it would be a problem for any older OS'es that aren't supported by .netcore3.0

[pvandervelde]

Darn it @JosephekcoL, you're not supposed to find bugs in my not-so-carefully written software :P

Let me know if you guys need a proper release