fix: copy bootstrap mcpp.exe before cleaning build dir

The self-host step was deleting the build/ directory which contained
the bootstrap binary, causing "No such file or directory" (exit 127).

Author: Sunrisepeak

.github/workflows/ci-windows.yml

@@ -237,10 +237,14 @@ jobs:
         shell: bash
         run: |
           echo "=== Self-host: using bootstrapped mcpp.exe to build mcpp ==="
+          # Save bootstrap binary before cleaning xmake artifacts
+          mkdir -p /tmp/mcpp-bootstrap
+          cp "$MCPP_BOOTSTRAP" /tmp/mcpp-bootstrap/mcpp.exe
+          MCPP_EXE="/tmp/mcpp-bootstrap/mcpp.exe"
+
           # Clean xmake artifacts so mcpp starts fresh
           rm -rf build xmake.lua .xmake
 
-          MCPP_EXE="$MCPP_BOOTSTRAP"
           echo "Bootstrap binary: $MCPP_EXE"
           "$MCPP_EXE" --version