GithubCI: don't use Microsoft.Maui/Gtk.slnf

As it doesn't exist in Mali/main.

.github/workflows/build-gtk.yml

@@ -33,7 +33,13 @@ jobs:
         run: |
             mv Directory.Build.Override.props.in Directory.Build.Override.props
             dotnet build Microsoft.Maui.BuildTasks.slnf
-            dotnet build -c Release Microsoft.Maui.Gtk.slnf
+            dotnet build -c Release src/BlazorWebView/src/Maui/Microsoft.AspNetCore.Components.WebView.Maui.csproj
+            dotnet build -c Release src/Essentials/src/Essentials.csproj
+            dotnet build -c Release src/Controls/src/Core/Controls.Core.csproj
+            dotnet build -c Release src/Controls/src/Xaml/Controls.Xaml.csproj
+            dotnet build -c Release src/Core/src/Core.csproj
+            dotnet build -c Release src/Compatibility/Core/src/Compatibility.csproj
+            dotnet build -c Release src/Controls/Foldable/src/Controls.Foldable.csproj
       - name: Pack MAUI
         run: |
             dotnet pack src/BlazorWebView/src/Maui/Microsoft.AspNetCore.Components.WebView.Maui.csproj --no-build --no-restore