Use aspnetcore runtime only instead of full SDK to run XHarness

[premun]

Context

To run XHarness through Helix SDK, we need ASP.NET runtime. However, in .NET 6 RC 1 the versions of the bundled NETCore.App runtime are somehow lower and the NETCore.App that is bundled with ASP.NET runtime cannot run XHarness CLI.

The version and type of .NET is set here:
https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.Helix/Sdk/tools/xharness-runner/XHarnessRunner.targets#L6

For this reason, we switched to using the full SDK in the XHarness workloads which is probably unnecessary in the future.

Problem details

.NET says it cannot find a runtime but the runtime seems to be present:

> dotnet --info
Host (useful for support):
  Version: 6.0.0-rc.1.21376.24
  Commit:  2053d653bf
.NET SDKs installed:
  No SDKs were found.
.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0-rc.1.21378.2 [/private/tmp/helix/working/AD8F0968/p/dotnet-cli/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0-rc.1.21376.24 [/private/tmp/helix/working/AD8F0968/p/dotnet-cli/shared/Microsoft.NETCore.App]

> dotnet --list-runtimes
Microsoft.AspNetCore.App 6.0.0-rc.1.21378.2 [/private/tmp/helix/working/AD8F0968/p/dotnet-cli/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.0-rc.1.21376.24 [/private/tmp/helix/working/AD8F0968/p/dotnet-cli/shared/Microsoft.NETCore.App]

dotnet exec /tmp/helix/working/AD8F0968/p/microsoft.dotnet.xharness.cli/1.0.0-dev/tools/net6.0/any/Microsoft.DotNet.XHarness.CLI.dll apple device ios-simulator-64
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '6.0.0-rc.1.21378.2' was not found.
  - The following frameworks were found:
      6.0.0-rc.1.21376.24 at [/private/tmp/helix/working/AD8F0968/p/dotnet-cli/shared/Microsoft.NETCore.App]

1) Installing aspnetcore runtime only

```
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '6.0.0-rc.1.21378.2' was not found.
  - The following frameworks were found:
      6.0.0-rc.1.21376.24 at [/mnt/d/github/xharness/artifacts/tmp/.dotnet/shared/Microsoft.NETCore.App]
```

```
> ./.dotnet/dotnet --list-runtimes
Microsoft.AspNetCore.App 6.0.0-rc.1.21378.2 [/mnt/d/github/xharness/artifacts/tmp/.dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.0-rc.1.21376.24 [/mnt/d/github/xharness/artifacts/tmp/.dotnet/shared/Microsoft.NETCore.App]
```

2) Installing dotnet runtime only

```
It was not possible to find any compatible framework version
The framework 'Microsoft.AspNetCore.App', version '6.0.0-rc.1.21378.2' was not found.
  - No frameworks were found.
```

```
> ./.dotnet/dotnet --list-runtimes
Microsoft.NETCore.App 6.0.0-rc.1.21378.2 [/mnt/d/github/xharness/artifacts/tmp/.dotnet/shared/Microsoft.NETCore.App]
```

3) Installing both dotnet+aspnetcore runtimes

Finally worked

Goal

As a workaround, we switched to using the full SDK in the XHarness workloads. Once we are past RC1 - maybe RC2, maybe full .NET 6 - start using the aspnetcore runtime only to run XHarness (basically revert the last change).

[riarenas]

I think this is another side effect of using an unreleased version of the sdk. I'm not sure there's a good way to see which runtimes are supposed to go with nightly unreleased versions. Once RC1 actually releases we need to move all repos to use the released version instead of the one we took to unblock nuget restores.

[premun]

Cool, so let's do it then. Should I put this todo next to the AspNetCoreRuntimeVersion that needs to be bumped together with the SDK so that people who do the bump notice?

[riarenas]

Is there a reason why it's not desirable to just always install the SDK? It seems like it's actually a goal for us to be able to use daily sdks in repos.

[premun]

I just assumed we save bandwidth since we need the runtime only. But if it doesn't matter much, I don't care which one we use.

[premun]

This is waiting for Arcade to switch onto .NET 7 preview 1