title: Can't build .Net 5 in pipeline
body: <p>I created a new solution 2 weeks ago using .Net 5 RC2. I've upgraded it to the new release (5.0.0). I'm trying to created a standard ASP.Net pipeline to build it, agent pool is Azure Pipelines, and agent specification I've tried both windows-2019 and vs2017-win2016. Both give the same errors as below
  Why is it trying to use 2.1?</p>
  <pre><code>Checking compatibility for System.Composition.AttributedModel 1.0.31 with .NETFramework,Version=v5.0 (win7-x86).
  Checking compatibility for System.Composition.Runtime 1.0.31 with .NETFramework,Version=v5.0 (win7-x86).
  Checking compatibility for System.Composition.TypedParts 1.0.31 with .NETFramework,Version=v5.0 (win7-x86).
  Incompatible packages: 4
  Committing restore...
  Generating MSBuild file D:\a\1\s\XXX\obj\XXX.csproj.nuget.g.props.
  Generating MSBuild file D:\a\1\s\XXX\obj\XXX.csproj.nuget.g.targets.
  Writing lock file to disk. Path: D:\a\1\s\XXX\obj\project.assets.json
  Writing cache file to disk. Path: D:\a\1\s\XXX\obj\XXX.csproj.nuget.cache
  Restore failed in 38.96 sec for D:\a\1\s\XXX\XXX.csproj.
  NuGet Config files used:
  D:\a\1\Nuget\tempNuGet_3.config
  Feeds used:
  https://api.nuget.org/v3/index.json
  Installed:
  71 package(s) to D:\a\1\s\XXX\XXX.csproj
  ##[error]The nuget command failed with exit code(1) and error(NU1202: Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Tools 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Tools 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Design 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Design 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Relational 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Relational 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Tools 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Tools 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Relational 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Relational 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Design 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Design 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  Errors in D:\a\1\s\XXX\XXX.csproj
  NU1202: Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Tools 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Tools 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Design 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Design 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Relational 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0). Package Microsoft.EntityFrameworkCore.Relational 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.SqlServer 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Tools 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Tools 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Relational 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Relational 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1)
  NU1202: Package Microsoft.EntityFrameworkCore.Design 5.0.0 is not compatible with net50 (.NETFramework,Version=v5.0) / win7-x86. Package Microsoft.EntityFrameworkCore.Design 5.0.0 supports: netstandard2.1 (.NETStandard,Version=v2.1))
  ##[error]Packages failed to restore
  Finishing: NuGet restore ```
  </code></pre>