If you're making changes to the SDK (that is, the dotnet/sdk repo) you can easily test VS or msbuild.exe with those changes by setting the DOTNET_MSBUILD_SDK_RESOLVER_SDKS_DIR environment variable.
After you build, find the generated Sdks directory. For example, if your repo is at D:\Projects\sdk, you'll find it at D:\Projects\sdk\bin\Debug\Sdks. Set the environment variable to point to this location:
set DOTNET_MSBUILD_SDK_RESOLVER_SDKS_DIR=D:\Projects\sdk\bin\Debug\Sdks
Now any instances of msbuild.exe or VS that inherit that setting will use your locally-produced SDK.