ci: multi-mirror GitHub clone for mingw third_party

windows-cross died cloning busybox-w32 through a global gitclone.com
insteadOf (HTTP 502). Try ghfast/ghproxy/gitclone/github per repo instead
of forcing one proxy for every fetch.

scripts/ci_windows_cross.sh

@@ -60,8 +60,9 @@ if ! printf '#include <stdio.h>\n' | x86_64-w64-mingw32-gcc -dM -E - | grep -qw
   exit 1
 fi
 
-# Third-party sources live on GitHub; prefer a CN-friendly rewrite.
-git config --global url."https://gitclone.com/github.com/".insteadOf "https://github.com/" || true
+# Do not set a global insteadOf: a single broken proxy (e.g. gitclone 502)
+# would take down every third_party clone. build_third_party_mingw.sh tries
+# several mirrors per repo instead.
 
 echo "==> bootstrap_windows.sh"
 exec bash scripts/bootstrap_windows.sh "$ROOT"