Arcade SDK InstallDotNetCore fails under .NET Framework MSBuild after System.Memory 4.6.3 update

- [x] This issue is blocking
This blocks dotnet/msbuild#14332 from consuming Arcade build `20260708.3`. All affected Windows full-framework bootstrap jobs fail before tests start.

Related to dotnet/msbuild#14332

### Context

`Microsoft.DotNet.Arcade.Sdk` version `10.0.0-beta.26358.3` fails when its `InstallDotNetCore` task runs under full-framework MSBuild. The package contains an inconsistent .NET Framework dependency set:

- `Microsoft.DotNet.Arcade.Sdk.dll` references `System.Memory, Version=4.0.5.0`.
- The bundled `System.Text.Json.dll` remains version `8.0.0.5` and references `System.Memory, Version=4.0.1.2`.

When `InstallDotNetCore.Execute()` calls `JsonDocument.Parse(bytes)`, method resolution fails in the isolated .NET Framework task AppDomain:

> `System.MissingMethodException: Method not found: 'System.Text.Json.JsonDocument System.Text.Json.JsonDocument.Parse(System.ReadOnlyMemory, System.Text.Json.JsonDocumentOptions)'`

### Copilot Investigation
The mismatch was introduced by dotnet/arcade#17060, which changed `SystemMemoryVersion` from `4.5.5` to `4.6.3`. The affected package was then flowed into dotnet/msbuild#14332. MSBuild itself contains no related code changes.

The Arcade package needs a coherent `System.Memory`/`System.Text.Json` dependency set for .NET Framework tasks. One possible approach is to keep the newer `System.Memory` dependency scoped to `Microsoft.DotNet.RemoteExecutor` rather than applying it to all Arcade projects.

- dotnet/msbuild build 1505477 fails in `Windows Full` during `cibuild_bootstrapped_msbuild.cmd`.
- dotnet/msbuild build 1505478 fails identically in the Windows quarantined-test job.
- Linux, macOS, Windows Core, and non-bootstrap Windows jobs pass.
- Package inspection confirmed that `10.0.0-beta.26324.4` had both the Arcade task and `System.Text.Json` referencing `System.Memory` `4.0.1.2`.
- Package inspection confirmed that `10.0.0-beta.26358.3` changed only the Arcade task reference to `System.Memory` `4.0.5.0`; the bundled `System.Text.Json` binary remained unchanged.
- No corrective change has been tested.

Start with Microsoft.DotNet.Arcade.Sdk and the InstallDotNetCore.Execute() path, then inspect the packaged System.Memory and System.Text.Json dependency versions. Reproduce the failure in the Windows full-framework bootstrap job and compare it with the passing package version 10.0.0-beta.26324.4. Done means the task runs under full-framework MSBuild and the affected Windows bootstrap jobs pass.

Written by the indexing model from the issue text.