Build sample fails on Linux: The reference assemblies for .NETFramework,Version=v4.6.1 were not found

[chrbsg]

Github issues should be used for bugs and feature requests. Use Stack Overflow for general "how-to" questions.

Version

$ dotnet --list-sdks
3.1.420 [/usr/share/dotnet/sdk]
6.0.301 [/usr/share/dotnet/sdk]

Describe the bug

I installed the recommended version of the dotnet sdk (dotnet-sdk-6.0) following instructions at https://docs.microsoft.com/en-us/dotnet/core/install/linux-ubuntu

then cloned this repo and did:

$ cd BotFramework-DirectLine-DotNet/samples/core-DirectLine/DirectLineClient
$ dotnet build DirectLineSampleClient.csproj 
Microsoft (R) Build Engine version 17.2.0+41abc5629 for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

  Determining projects to restore...
  Nothing to do. None of the projects specified contain packages to restore.
/usr/share/dotnet/sdk/6.0.301/Microsoft.Common.CurrentVersion.targets(1221,5): error MSB3644: The reference assemblies for .NETFramework,Version=v4.6.1 were not found. To resolve this, install the Developer Pack (SDK/Targeting Pack) for this framework version or retarget your application. You can download .NET Framework Developer Packs at https://aka.ms/msbuild/developerpacks [/home/chrb/git/BotFramework-DirectLine-DotNet/samples/core-DirectLine/DirectLineClient/DirectLineSampleClient.csproj]

Build FAILED.

I can't see any way to install dotnet-sdk-4 in Ubuntu - the MS repos only have dotnet package versions 2.1, 3.1, 5.0, and 6.0:

$ apt-cache search dotnet-sdk
dotnet-sdk-2.1 - Microsoft .NET Core SDK 2.1.818
dotnet-sdk-3.1 - Microsoft .NET Core SDK 3.1.420
dotnet-sdk-5.0 - Microsoft .NET SDK 5.0.408
dotnet-sdk-6.0 - Microsoft .NET SDK 6.0.301

To Reproduce

Steps to reproduce the behavior:
As above

Expected behavior

Should compile ok with latest recommended SDK version

Screenshots

If applicable, add screenshots to help explain your problem.

Additional context

Add any other context about the problem here.

[bug]