Building solution with forced out-of-proc nodes fails

📝 I'm not sure we're going to be very motivated to fix this, but it cost me a lot of time debugging bootstrapped-build failures in #3365, so I'm filing it.

### Steps to reproduce

Build a solution using the MSBuild task

```xml

```

While using `MSBUILDNOINPROCNODE`, this results in an error

```
S:\msbuild\some.sln(2,1): error MSB4025: The project file could not be loaded. Data at the root level is invalid. Line 2, position 1.
```

### Why?

Solution metaprojects are built based on an in-memory instance which isn't transferable. See

https://github.com/Microsoft/msbuild/blob/2a012e653766eb261b09b29b3106f4eb57a7f61d/src/Build/BackEnd/Components/Scheduler/Scheduler.cs#L1254-L1260

So when forced to out-of-proc nodes, the solution build fails.

### Environment data
`msbuild /version` output: `15.8.139-preview+g5951330944 for .NET Framework` (also on Core)