Outputs not assigned from MSBuild task when ContinueOnError=true and BuildInParallel=true

This is the result of a continuing investigation into intermittent failures of an internal build.

### Steps to reproduce

Build multiple projects, one of which fails, in a single MSBuild task invocation with both `BuildInParallel="true"` and `ContinueOnError="true"`.

[continueonerror.zip](https://github.com/Microsoft/msbuild/files/1733212/continueonerror.zip)

### Expected behavior

Regardless of the value of `BuildInParallel`, the returned item should be populated with the success cases:

```
S:\work\intermittent-devdiv-failure\continueonerror>msbuild /nologo /v:m entry.proj /p:BuildInParallel=false
S:\work\intermittent-devdiv-failure\continueonerror\fail.proj(3,3): error : Failure!
Returned value: success
```

### Actual behavior

When `BuildInParallel=true` (even on a single-proc build), no output is assigned

```
S:\work\intermittent-devdiv-failure\continueonerror>msbuild /nologo /v:m entry.proj /p:BuildInParallel=true
S:\work\intermittent-devdiv-failure\continueonerror\fail.proj(3,3): error : Failure!
Returned value:
```

### Environment data
`msbuild /version` output: `15.6.81.24225 for .NET Framework` on my machine; `15.5.179.9764 for .NET Framework` internally

Also repros on `4.7.2556.0`, so this is not a recent regression.