ci: install the Windows .NET SDK per architecture

The UseDotNet@2 task has no architecture input and mis-detects
Windows-on-Arm agents as win-x86, installing the 32-bit SDK. Publishing
win-arm64 from an x86 host is then treated as Native AOT
cross-compilation (only x64->arm64 is supported), so the ILCompiler step
fails demanding a runtime.win-arm64.Microsoft.DotNet.ILCompiler package.
The x86/x64 legs pass because they run on Intel agents, where the SDK
targets win-x64 natively and win-x86 via bundled cross-support.

Acquire the SDK with the official dotnet-install script instead, keyed
on the matrix leg's poolArch, so every Windows agent gets a host SDK
matching its architecture and publishes natively. The macOS and Linux
legs keep UseDotNet@2; the mis-detection is specific to Windows on Arm.

Assisted-by: Claude Opus 4.8
Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

.azure-pipelines/release.yml

@@ -134,11 +134,21 @@ extends:
                     artifactName: '${{ dim.runtime }}'
               steps:
                 - checkout: self
-                - task: UseDotNet@2
-                  displayName: 'Use .NET 10 SDK'
+                - task: PowerShell@2
+                  displayName: 'Install .NET 10 SDK'
                   inputs:
-                    packageType: sdk
-                    version: '10.x'
+                    targetType: inline
+                    script: |
+                      # UseDotNet@2 mis-detects Windows-on-Arm agents as win-x86
+                      # and installs the 32-bit SDK, which breaks the win-arm64
+                      # AOT publish. Call dotnet-install directly, keyed on the
+                      # agent architecture, so each leg gets a matching host SDK.
+                      $ProgressPreference = 'SilentlyContinue'
+                      $installDir = Join-Path $env:AGENT_TEMPDIRECTORY 'dotnet'
+                      $installer = Join-Path $env:AGENT_TEMPDIRECTORY 'dotnet-install.ps1'
+                      Invoke-WebRequest -Uri 'https://dot.net/v1/dotnet-install.ps1' -OutFile $installer
+                      & $installer -Channel 10.0 -Architecture ${{ dim.poolArch }} -InstallDir $installDir
+                      Write-Host "##vso[task.prependpath]$installDir"
                 # Native AOT links the published binary with the MSVC linker,
                 # which the hosted images do not ship by default. Install the
                 # VC++ build tools for the agent's architecture first.