Support installing multiple versions of .NET Core SxS

I would like to use this action to setup .NET Core for some class libraries I support. I like to run tests on multiple versions of .NET Core, but it appears I cannot use this task to install multiple versions of .NET Core.

Repro

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/setup-dotnet@v1
      with:
        version: '2.1.801'        
    - run: dotnet --info
    - uses: actions/setup-dotnet@v1
      with:
        version: '3.0.100-preview8-013656'
    - run: dotnet --info

Expected

Both .NET Core 2.1 and 3.0 are installed and available when running dotnet commands.

Actual

Usages of actions/setup-dotnet are not additive; the last one wins, so only 3.0.100-preview8-013656 is available.

logs.zip

[natemcmaster]

Here is one use case (there are more, but this is the most common). Consider, for instance, a test project file like this:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
     <TargetFrameworks>netcoreapp3.0;netcoreapp2.2;netcoreapp2.1</TargetFrameworks>
  </PropertyGroup>
  <ItemGroup>
     <PackageReference Include="xunit" Version="2.4.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.0.0" />
  </ItemGroup>
</Project>

When you run dotnet test for this project, the test runner will launch tests for the three versions of .NET Core listed, 2.1, 2.2, and 3.0. If only 3.0 is installed, the 2.1 and 2.2 test runs would fail.

The way to make this work is to install all versions of .NET Core into the same root folder.

[alaatm]

The following can be used as a workaround until this gets implemented:

  - name: Setup .net core 2.2
    uses: actions/setup-dotnet@v1
    with:
      dotnet-version: 2.2.402

  - name: Setup .net core 3.0
    uses: actions/setup-dotnet@v1
    with:
      dotnet-version: 3.0.100

  - name: .net SxS
    run: |
      rsync -a ${DOTNET_ROOT/3.0.100/2.2.402}/* $DOTNET_ROOT/