Solution meta-project doesn't propagate target return values for custom targets

### Steps to reproduce

Project file a.csproj
```xml

net46

```

Solution A.sln containing a.csproj.

Project file x.proj
```xml

```

Command line
```
msbuild x.proj
```
### Expected behavior

`Things: 'A;B;C'`

### Actual behavior

`Things: ''`

### Environment data
`msbuild /version` output:

```
Microsoft (R) Build Engine version 15.8.166+gd4e8d81a88 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.
```