Native .NET 7 source build on s390x

### Describe the Problem

I'm trying to use the source build instructions for building .NET 7 on s390x.
This fails almost immediately with:

```
installer$ ./build.sh /p:ArcadeBuildTarball=true /p:TarballDir=$PWD/source-build
Downloading 'https://dotnet.microsoft.com/download/dotnet/scripts/v1/dotnet-install.sh'
...
dotnet-install: Extracting zip from https://dotnetcli.azureedge.net/dotnet/Sdk/7.0.100-preview.5.22307.18/dotnet-sdk-7.0.100-preview.5.22307.18-linux-x64.tar.gz
...
dotnet-install: Extracting zip from https://dotnetcli.azureedge.net/dotnet/Runtime/2.1.0/dotnet-runtime-2.1.0-linux-x64.tar.gz
...
{redacted}/installer/eng/common/tools.sh: line 477: {redacted}/installer//.dotnet/dotnet: cannot execute binary file: Exec format error
```

This is essentially https://github.com/dotnet/installer/issues/13022, however, the `7.0.100-preview.5.22307.18` part can be circumvented by extracting the manually built installer into `.dotnet`. The `2.1.0` part is not that easy though, since AFAICS 2.1.0 is EOL (https://dotnet.microsoft.com/en-us/download/dotnet/2.1 + https://www.nuget.org/packages/Microsoft.NETCore.App/2.1.0) and will never be supported on s390x.

### Describe the Solution

`./build.sh /p:ArcadeBuildTarball=true /p:TarballDir=$PWD/source-build` should not require dotnet-runtime-2.1.0.