Tests fail for Linux local build on latest commits: "The framework 'Microsoft.NETCore.App', version '2.1.0' was not found."

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

When I try to clone latest arcade and build with tests, it fails. Repro command:

```
docker run -it --rm mcr.microsoft.com/dotnet/core/sdk:3.1 bash -c '
git clone https://github.com/dotnet/arcade
cd arcade
git checkout 342467b # Latest as of writing, for reproducibility
./build.sh -test'
```

Also repros with the container used in the PR validation build (no global .NET installed):

https://github.com/dotnet/arcade/blob/342467b3c4af696dcffe746ee6239340461db753/azure-pipelines.yml#L20-L21

```
docker run -it --rm microsoft/dotnet-buildtools-prereqs:ubuntu-14.04-cross-0cd4667-20170319080304 bash -c '
git clone https://github.com/dotnet/arcade
cd arcade
git checkout 342467b # Latest as of writing, for reproducibility
./build.sh -test'
```

They hit errors at the end of the build:

```
...
Running tests: /arcade/artifacts/bin/Microsoft.DotNet.SignTool.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.SignTool.Tests.dll [netcoreapp2.1|x64]
Running tests: /arcade/artifacts/bin/Microsoft.DotNet.RemoteExecutor.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.RemoteExecutor.Tests.dll [netcoreapp2.1|x64]
Running tests: /arcade/artifacts/bin/Microsoft.DotNet.Build.Tasks.Feed.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.Build.Tasks.Feed.Tests.dll [netcoreapp2.1|x64]
XUnit : error : Tests failed: /arcade/artifacts/log/Debug/Microsoft.DotNet.Build.Tasks.Feed.Tests_netcoreapp2.1_x64.log [netcoreapp2.1|x64] [/arcade/src/Microsoft.DotNet.Build.Tasks.Feed.Tests/Microsoft.DotNet.Build.Tasks.Feed.Tests.csproj]
XUnit : error : Tests failed: /arcade/artifacts/log/Debug/Microsoft.DotNet.SignTool.Tests_netcoreapp2.1_x64.log [netcoreapp2.1|x64] [/arcade/src/Microsoft.DotNet.SignTool.Tests/Microsoft.DotNet.SignTool.Tests.csproj]

...

$ cat artifacts/log/Debug/Microsoft.DotNet.Build.Tasks.Feed.Tests_netcoreapp2.1_x64.log
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
- The following frameworks were found:
5.0.0-preview.6.20305.6 at [/arcade/.dotnet/shared/Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
- https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=2.1.0&arch=x64&rid=ubuntu.14.04-x64
=== COMMAND LINE ===
"/arcade/.dotnet/dotnet" exec --depsfile "/arcade/artifacts/bin/Microsoft.DotNet.Build.Tasks.Feed.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.Build.Tasks.Feed.Tests.deps.json" --runtimeconfig "/arcade/artifacts/bin/Microsoft.DotNet.Build.Tasks.Feed.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.Build.Tasks.Feed.Tests.runtimeconfig.json" "/root/.nuget/packages/xunit.runner.console/2.4.1/tools/netcoreapp2.0/xunit.console.dll" "/arcade/artifacts/bin/Microsoft.DotNet.Build.Tasks.Feed.Tests/Debug/netcoreapp2.1/Microsoft.DotNet.Build.Tasks.Feed.Tests.dll" -noautoreporters -xml "/arcade/artifacts/TestResults/Debug/Microsoft.DotNet.Build.Tasks.Feed.Tests_netcoreapp2.1_x64.xml" -html "/arcade/artifacts/TestResults/Debug/Microsoft.DotNet.Build.Tasks.Feed.Tests_netcoreapp2.1_x64.html" > "/arcade/artifacts/log/Debug/Microsoft.DotNet.Build.Tasks.Feed.Tests_netcoreapp2.1_x64.log" 2>&1
```