azure-pipelines-tasks: [BUG] UseDotNet@2 always downloads SDK, even when it is already installed

Question, Bug, or Feature?
Type: BUG

Enter Task Name: Use .NET core v2

Environment

• Server - Azure Pipelines or TFS on-premises? Azure pipeline

  • If using Azure Pipelines, provide the account name, team project name, build definition name/build number: Not posting private information to a public place. You shouldnt really be asking us to divulge that.

Issue Description

In the WIndows 2019 hosted agent image inventory, it lists the versions of the .net core SDK that are available, including 3.1.410

In the build pipeline, there is a Use .NET Core v2 task that is configured to use .net 3.x (we are skipping .net5 and will look to target 6 when its released)

When the pipeline runs, it determines it needs to use version 3.1.410 as it is the latest version that matches the configured spec. Since this version is also already installed on the agent already, I would expect that this installation is what is used. However, per the task logs the pipeline downloads and installs this SDK version. This task takes about 60 seconds to complete, when it should take less time as it should not need to download and install anything.

Starting: Use .NET Core sdk 3.x
==============================================================================
Task         : Use .NET Core
Description  : Acquires a specific version of the .NET Core SDK from the internet or the local cache and adds it to the PATH. Use this task to change the version of .NET Core used in subsequent tasks. Additionally provides proxy support.
Version      : 2.184.0
Author       : Microsoft Corporation
Help         : https://aka.ms/AA4xgy0
==============================================================================
Tool to install: .NET Core sdk version 3.x.
Found version 3.1.410 in channel 3.1 for user specified version spec: 3.x
Version 3.1.410 was not found in cache.
Getting URL to download .NET Core sdk version: 3.1.410.
Detecting OS platform to find correct download package for the OS.
C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe -NoLogo -Sta -NoProfile -NonInteractive -ExecutionPolicy Unrestricted -Command "& 'D:\a\_tasks\UseDotNet_b0ce7256-7898-45d3-9cb5-176b752bfea6\2.184.0\externals\get-os-platform.ps1'"
Primary:win-x64
Detected platform (Primary): win-x64
Downloading: https://download.visualstudio.microsoft.com/download/pr/94cd3943-95b4-4d86-a3ec-07f434b460b9/e0db650eba4bf8b3ec8ef99312a66047/dotnet-sdk-3.1.410-win-x64.zip
Extracting downloaded package D:\a\_temp\d16cf416-68be-4acb-92d1-cf4c4f49cf40.
Extracting archive
C:\Windows\system32\chcp.com 65001
Active code page: 65001
C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe -NoLogo -Sta -NoProfile -NonInteractive -ExecutionPolicy Unrestricted -Command "$ErrorActionPreference = 'Stop' ; try { Add-Type -AssemblyName System.IO.Compression.FileSystem } catch { } ; [System.IO.Compression.ZipFile]::ExtractToDirectory('D:\a\_temp\d16cf416-68be-4acb-92d1-cf4c4f49cf40', 'D:\a\_temp\a9e3')"
Successfully installed .NET Core sdk version 3.1.410.
Creating global tool path and pre-pending to PATH.
Finishing: Use .NET Core sdk 3.x

Is the agent installation manifest correct and this task is bugged, or is the agent installation manifest incorrect?

I realize this bug is a money maker for you (because by not fixing this, builds take longer, which increases the number of concurrent hosted agents that we have to pay for just to keep up), but its not working as advertised.