Microsoft.CodeAnalysis fails to load on `dotnet build` in a custom MSBuild task

moved from https://github.com/dotnet/roslyn/issues/26569 on behalf of @pbalaga

**Version Used**:
[Microsoft.CodeAnalysis 2.8.0](https://www.nuget.org/packages/Microsoft.CodeAnalysis/)

`dotnet --info`:
```
.NET Command Line Tools (2.1.105)

Product Information:
Version: 2.1.105
Commit SHA-1 hash: 141cc8d976

Runtime Environment:
OS Name: Windows
OS Version: 10.0.16299
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.1.105\

Microsoft .NET Core Shared Framework Host

Version : 2.0.7
Build : 2d61d0b043915bc948ebf98836fefe9ba942be11
```

**Steps to Reproduce**:

1. `git clone https://github.com/pbalaga/TaskWithCodeAnalysisFailure.git`
2. `cd TaskWithCodeAnalysisFailure\TaskConsumerProject`
3. `dotnet build`

Important points about how the above repro repository was built:
- TaskLibrary.csproj
* define a custom MsBuild task: `SampleTask`
* `true` in order to copy NuGet references to bin folder
- TaskConsumerProject.csproj
* invoke SampleTask from TaskLibrary's bin folder:

```

```

**Expected Behavior**:
Project should build without errors.

**Actual Behavior**:
Build fails with the below error:
>TaskConsumerProject\TaskConsumerProject.csproj(11,5): error MSB4018: System.IO.FileLoadException: Could not load file or assembly 'Microsoft.CodeAnalysis, Version=2.8.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'. Could not find or load a specific file. (Exception from HRESULT: 0x80131621)

**Remarks**:
- the same project builds without errors in Visual Studio 2017 and via MsBuild CLI
- commenting out [the line](https://github.com/pbalaga/TaskWithCodeAnalysisFailure/blob/1f440e36f2ca0648d3b7624fefb03a173ea947d0/TaskLibrary/Task.cs#L11) containing Microsoft.CodeAnalysis usage makes the problem disappear
- all projects are targeting netstandard2.0, no multi-targeting (so not the same issue as https://github.com/dotnet/cli/issues/7510)
- I managed to attach a debugger to `SampleTask` before it throws an exception, but there is no more detail presented than in console output
- two additional dependencies were added to `SampleTask`: a project dependency and a NuGet dependency on a package different than Microsoft.CodeAnalysis to rule out possibility that this is a more general `dotnet`'s problem with dependency resolution. Only using Microsoft.CodeAnalysis causes the build to fail
- I tried other versions of Microsoft.CodeAnalysis, specifically: `2.8.0-beta4` and `2.7.0`, both of which produce exactly the same results