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"
 }