Fix CI build failures for WinAppSDK project

[Copilot]

The build workflow failed because it targeted the full solution, which pulls in the .wapproj packaging project — incompatible with dotnet build (missing Microsoft.DesktopBridge.props) and broken with msbuild too (CsWinRT resolved system SDK 10.0.22621.0 instead of the NuGet-provided 10.0.26100.38, causing a missing Platform.xml error).

Changes

• Build only the main .csproj instead of the solution, bypassing the .wapproj entirely (MSIX packaging is irrelevant for CI validation)
• Scoped dotnet restore to the same .csproj
• Added -p:Platform=x64 since WinAppSDK/WinUI projects do not support AnyCPU

- name: Restore dependencies
  run: dotnet restore "Simple Icon File Maker/Simple Icon File Maker/Simple Icon File Maker.csproj"

- name: Build
  run: dotnet build "Simple Icon File Maker/Simple Icon File Maker/Simple Icon File Maker.csproj" --configuration Release --no-restore -p:Platform=x64

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