NuGet restore fails with NU1101 for arch-x64 RID: Microsoft.NETCore.App.Host.arch-x64 not found

### Describe the bug

On Arch Linux-based distributions (Arch, CachyOS, Manjaro, etc.), `dotnet restore` and `dotnet build` fail because NuGet cannot find packages for the `arch-x64` Runtime Identifier (RID), even though this RID is officially defined in the SDK's RuntimeIdentifierGraph.json.
To Reproduce
**System:** CachyOS Linux (Arch-based)
**SDK Version:** 10.0.101

1. Create any .NET project: `dotnet new console`
2. Attempt to build: `dotnet build`
Error Output
error NU1101: Unable to find package Microsoft.NETCore.App.Host.arch-x64. No packages exist with this id in source(s): nuget.org

## Root Cause Analysis

The SDK correctly identifies the system as `arch-x64`:

- `/usr/share/dotnet/sdk/10.0.100/Microsoft.NETCoreSdk.BundledVersions.props` line 28:
```xml
arch-x64
linux-x64
```
- /usr/share/dotnet/sdk/10.0.100/RuntimeIdentifierGraph.json contains:
arch-x64: {
#import: [ arch, linux-x64 ]
}
- arch-x64 is included in the official AppHostRuntimeIdentifiers list in BundledVersions.props
However: NuGet restore does not follow the RID fallback chain from arch-x64 → linux-x64, and packages are not published with arch-x64 variants on NuGet.org.
Expected behavior
NuGet should automatically fall back from arch-x64 to linux-x64 during package restoration, as defined in the RID graph's #import directive.
Actual behavior
NuGet looks for Microsoft.NETCore.App.Host.arch-x64 specifically and fails, despite the fallback being defined.
Environment
.NET SDK:
Version: 10.0.100
Commit: b0f34d51fc
Runtime Environment:
OS Name: cachyos
OS Platform: Linux
RID: arch-x64

dotnet --info shows RID as: arch-x64
Workaround
Manually override the RID in the project file or build properties:

linux-x64

Impact
This affects all Arch Linux users (Arch, Manjaro, CachyOS, EndeavourOS, etc.) on .NET 10.0, making it impossible to build projects without workarounds.
Related Issues

- #52058 - Similar Prune Package data issue on Arch Linux with .NET 10
- #17333 - Same issue reported in .NET 5 (closed without resolution)
Notes
This appears to be a regression or incomplete implementation where:

1. The SDK properly defines and detects arch-x64 as a valid RID
2. The RID graph correctly maps the fallback to linux-x64
3. But NuGet restore logic doesn't honor the fallback chain during package resolution