[CI] New build action + Build framework on linux (#2848)

* Create dotnet.yml

* Update dotnet.yml

* Update dotnet.yml

* Update dotnet.yml

* pragma disable some obsolete warnings

* Update dotnet.yml

* Update dotnet.yml

* Update dotnet.yml

* update vars for windows

* cache? dotnet

* oops forgot to save

* add concurrency

* test deploy

* fix?

* disable caching

* Update dotnet.yml

* Update dotnet.yml

* Update dotnet.yml

* ...

* Update dotnet.yml

* it should build now. maybe. probably

* huh?

* huh x2

* disable deploy on PRs / enable cache

* publish test results

* Update dotnet.yml

* remove cache cuz it's useless

* Update dotnet.yml

* push to github packages cuz why not

* add toggle vars for pushes

* Update dotnet.yml

* uncomment release since there's a toggle now

src/Discord.Net.Core/Discord.Net.Core.csproj

@@ -5,8 +5,7 @@
     <AssemblyName>Discord.Net.Core</AssemblyName>
     <RootNamespace>Discord</RootNamespace>
     <Description>The core components for the Discord.Net library.</Description>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">net6.0;net5.0;net461;netstandard2.0;netstandard2.1</TargetFrameworks>
-    <TargetFrameworks Condition=" '$(OS)' != 'Windows_NT' ">net6.0;net5.0;netstandard2.0;netstandard2.1</TargetFrameworks>
+    <TargetFrameworks>net6.0;net5.0;net461;netstandard2.0;netstandard2.1</TargetFrameworks>
     <WarningLevel>5</WarningLevel>
     <TreatWarningsAsErrors>True</TreatWarningsAsErrors>
   </PropertyGroup>
@@ -29,4 +28,7 @@
   <ItemGroup Condition=" '$(TargetFramework)' != 'net461'">
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>
+ <ItemGroup Condition="$(TargetFramework.StartsWith('net4')) AND '$(MSBuildRuntimeType)' == 'Core' AND '$(OS)' != 'Windows_NT'">
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" PrivateAssets="All" />
+  </ItemGroup>
 </Project>