Incremental build does not work for C++ project when using Build Solution command

We want to extend our C++ project with the support for **proto** files. We defined **ProjectSchemaDefinitions** for ProtoBuf and Visual Studio can recognize such files.

We extended the **BeforeBuildGenerateSourcesTargets** property to execute protoc compiler when C++ project is generating source code files (see Microsoft.BuildSteps.Targets from VC160 toolset).

Out target is correctly called when we run **Rebuild** or the **vcxproj** file is changed. We checked the Inputs and Outputs definitions for the GrpcCompileForCpp target and incremental build detects if proto file changed and GrpcCompileForCpp should be run.

When we use **Build Solution** only, Visual Studio will immediately return **All outputs are up-to-date.** and it does not even run the msbuild. This causes issues when we change proto files - they won't be compiled unless we run Rebuild.

### Steps to reproduce

Project file
```xml

$(BeforeBuildGenerateSourcesTargets);GrpcCompileForCpp
$(MSBuildThisFileDirectory)




"$(ProtocFullPath)" --cpp_out=$(ProtoOutputPath) --grpc_out=$(ProtoOutputPath) --plugin=protoc-gen-grpc=$(GrpcCppPluginFullPath) -I $(ProtoOutputPath) @(ProtoBuf->'%(FullPath)', ' ')

```

Directory contents:
```
/
- Server.proto
- ConsoleApplication4.vcxproj
```

Run **Build** → **Build Solution** command from Visual Studio.

### Expected behavior

Visual Studio will run msbuild and it will detect the **proto** file has changed and it will generate new source code.

### Actual behavior

Visual Studio reports **All outputs are up-to-date.**

### Environment data

Windows 10 1903
Visual Studio 2019
msbuild 16.2.37902.0