Fix dotnet tool-manifest creation with explicit --output flag

When a tool manifest exists in a parent directory, dotnet new tool-manifest
skips creation without error. Use --output to explicitly specify the target
directory so the manifest is always created where expected.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gfraiteur

src/PostSharp.Engineering.BuildTools/Utilities/DotNetTool.cs

@@ -69,10 +69,12 @@ public bool Install( BuildContext context )
                 // 1. Create the dotnet tool manifest.
                 if ( !File.Exists( configFilePath ) )
                 {
+                    // Use --output to explicitly specify where to create the manifest.
+                    // Without this, dotnet will skip creation if a manifest exists in a parent directory.
                     if ( !ToolInvocationHelper.InvokeTool(
                             context.Console,
                             "dotnet",
-                            $"new tool-manifest",
+                            $"new tool-manifest --output \"{baseDirectory}\"",
                             baseDirectory ) )
                     {
                         return false;