Action fails for C# 8

I have a C# 8 project that uses C# 8 syntax

I have a simple action to build it:

name: ASP.NET Core CI

on: [push]

jobs:
  build:

    runs-on: ubuntu-latest
    
    steps:
    - uses: actions/checkout@v2
    - name: Setup .NET Core
      uses: actions/setup-dotnet@v1
      with:
        dotnet-version: 2.2.108
    - name: Build with dotnet
      run: dotnet build --configuration Release

However, this throws an error whenever it encounters C# 8 syntax.

I should be able to force the lang version in the .csproj file with <LangVersion>8.0</LangVersion>, but this errors with CSC : error CS1617: Invalid option '8.0' for /langversion. Use '/langversion:?' to list supported values. [/home/runner/work/...myproject.csproj]

How do I upgrade this action to compile a C#8 project?

[plioi]

If that’s not actually evidence of anything wrong with setup-dotnet, I’ll chalk it up to my Solution being weird 😃. That should be a temporary setback for me, and I’ll submit a fresh Issue if I still have trouble once all projects target netcoreapp3.1+.

[plioi]

I wondered if that was the case, so I tried again with two invocations of setup-dotnet:

https://github.com/fixie/fixie/runs/704723182

Using the following definition:

name: CI
on: [push, pull_request]
jobs:
  ci:
    runs-on: ${{ matrix.os }}
    strategy:
      matrix:
        os: [windows-latest, macos-latest, ubuntu-latest]
    steps:
    - name: Checkout
      uses: actions/checkout@v2
    - name: Setup .NET Core 2 (Deprecated `dotnet fixie` Support)
      uses: actions/setup-dotnet@v1
      with:
        dotnet-version: 2.1.806
    - name: Setup .NET Core 3
      uses: actions/setup-dotnet@v1
      with:
        dotnet-version: 3.1.201
    - run: pwsh ./build.ps1

Ubuntu fails with errors about missing C# 8. Both invocations of setup-dotnet appear to succeed.