debug: dump build.ninja on self-host failure

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -274,7 +274,11 @@ jobs:
             ls "$MCPP_XPKGS/xim-x-llvm/20.1.7/bin/" | head -5
           fi
 
-          "$MCPP_EXE" build
+          "$MCPP_EXE" build || {
+            echo "=== build.ninja (first 50 lines) ==="
+            find target -name "build.ninja" -exec head -50 {} \; 2>/dev/null
+            exit 1
+          }
 
           # Find the self-hosted binary
           SELF_MCPP=$(find target -name "mcpp.exe" -path "*/bin/*" | head -1)