Fix DependencyInjectionPluginFramework in build.targets

[AdCodicem]

This PR fix the following issue occurring when using the plugin in a .NET framework project :
error MSB3030: Could not copy the file "C:\Users\<username>\.nuget\packages\solidtoken.specflow.dependencyinjection\0.4.1\lib\net45\SolidToken.SpecFlow.DependencyInjection.SpecFlowPlugin.dll" because it was not found.

[AdCodicem]

I created my own plugin for using Microsoft DependencyInjection with SpecFlow until I saw yours, so here is a small PR which fix an error which occured on one of my project.
I tested it locally

[mbhoek]

Thank you for your PR. This makes me realise we need tests in place for both .NET Core and .NET Framework. In order to do that, I would like to recreate the error you described.
Can I ask you what version of .NET Framework you used?

[AdCodicem]

My project is in .NET framework 4.7.2 (net472)

[AdCodicem]

Hi @mbhoek,
I updated my PR to include the tests as well.
As explained in the commit comment :

• I used Specflow 3.3.22-beta (only for test project) to resolve the following issue SpecFlow doesn't work with .NET Core 3.1.200 SDK SpecFlowOSS/SpecFlow#1912
  Its an issue which throw an error when MSBuild try to generate the .feature.cs files happening with .NET Core 3.1
• I mutualized the Specflow references in the test project for the same reasons.

I believe the tests remain relevant even with these modifications.