Azure-pipelines-tasks: DotNetCoreCLI@2 doesn't work with C# 8

[JnxF]

Required Information

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Question, Bug, or Feature?
Type: Bug

Enter Task Name: DotNetCoreCLI@2

Environment:

Azure Pipelines (/ondfisk/C5/14
Hosted on Azure DevOps

Issue Description

I'm trying to build my .NET Standard 2.0 project with Azure pipelines,
```steps:

• task: DotNetCoreCLI@2
  inputs:
  command: build
  arguments: /P:SourceLinkCreate=false --configuration $(buildConfiguration)
  displayName: Build

It was working perfectly until I added support for Nullable Reference Types checking and C# 8 in my solution .csproj file:

8.0
enable

### Task logs

CSC : error CS1617: Invalid option '8.0' for /langversion. Use '/langversion:?' to list supported values. [d:\a\1\s\C5\C5.csproj]

Build FAILED.

CSC : error CS1617: Invalid option '8.0' for /langversion. Use '/langversion:?' to list supported values. [d:\a\1\s\C5\C5.csproj]
0 Warning(s)
1 Error(s)

Time Elapsed 00:01:45.50

[error]Error: C:\Program Files\dotnet\dotnet.exe failed with return code: 1

[error]Dotnet command failed with non-zero exit code on the following projects :

[section]Finishing: Build

```

[Flavien]

Did you find a solution to this?

[JnxF]

Yes, the solution came by adding

```steps:

• task: DotNetCoreInstaller@0
  inputs:
  packageType: 'sdk'
  version: '3.0.100-preview3-010431'
  displayName: Install .NET Core 3