ci: install x86 .NET 10 runtime on Windows x86 builds

The x86 test host requires an x86 .NET runtime, which isn't
pre-installed on the GitHub runner, nor can the
actions/setup-dotnet action install it (only the native arch SDK is
installed).

Install it manually using the official installer script.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

.github/workflows/continuous-integration.yml

@@ -32,6 +32,15 @@ jobs:
       with:
         dotnet-version: 10.0.x
 
+    # The x86 test host requires an x86 .NET runtime, which isn't pre-installed
+    # on the runner, nor can the actions/setup-dotnet action install it.
+    # Install it manually so tests can run.
+    - name: Setup .NET (x86)
+      if: matrix.runtime == 'win-x86'
+      run: |
+        Invoke-WebRequest 'https://dot.net/v1/dotnet-install.ps1' -OutFile dotnet-install.ps1
+        ./dotnet-install.ps1 -Channel 10.0 -Architecture x86 -InstallDir 'C:\Program Files (x86)\dotnet'
+
     - name: Install dependencies
       run: dotnet restore