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/build_third_party_mingw.sh

@@ -22,11 +22,35 @@ need_cmd git
 need_cmd make
 need_cmd "${T}gcc"
 
+# Clone a GitHub repo with CN-friendly mirrors; fall back if one proxy 502s.
+github_mirrors() { # https://github.com/org/repo[.git] -> print candidate clone URLs
+  local url="$1" path
+  path="${url#https://github.com/}"
+  path="${path%.git}"
+  printf '%s\n' \
+    "https://ghfast.top/https://github.com/${path}.git" \
+    "https://ghproxy.net/https://github.com/${path}.git" \
+    "https://gitclone.com/github.com/${path}.git" \
+    "https://github.com/${path}.git"
+}
+
 checkout() { # url dir rev
-  local url="$1" dir="$2" rev="$3"
-  [[ -d "$dir/.git" ]] || git clone -q --filter=blob:none "$url" "$dir"
+  local url="$1" dir="$2" rev="$3" m
+  if [[ ! -d "$dir/.git" ]]; then
+    local ok=0
+    while IFS= read -r m; do
+      echo "  clone $m"
+      rm -rf "$dir"
+      if git clone -q --filter=blob:none "$m" "$dir"; then
+        ok=1
+        break
+      fi
+    done < <(github_mirrors "$url")
+    [[ $ok -eq 1 ]] || { echo "clone failed: $url" >&2; exit 1; }
+  fi
   if ! git -C "$dir" rev-parse -q --verify "$rev^{commit}" >/dev/null; then
-    git -C "$dir" fetch -q --tags origin
+    git -C "$dir" fetch -q --tags origin || true
+    git -C "$dir" fetch -q origin "$rev" || true
   fi
   git -C "$dir" checkout -q --detach "$rev"
 }

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"