ci: pin sqlite3 + pre-fetch onnxruntime offline

Run 11 reached the build and revealed two issues:
- Runner re-resolved sqlite3 to 3.3.3 (cn mirror), mismatching the
  vendored 3.3.2 binary hash. Pin it via dependency_overrides.
- flutter_onnxruntime's github download of ONNX Runtime failed.
  Pre-fetch it via a China-accessible proxy into the plugin's
  expected build path so it skips the download and bundles the dll.

pubspec.lock

@@ -958,7 +958,7 @@ packages:
     source: hosted
     version: "2.4.0"
   sqlite3:
-    dependency: transitive
+    dependency: "direct overridden"
     description:
       name: sqlite3
       sha256: "9488c7d2cdb1091c91cacf7e207cff81b28bff8e366f042bad3afe7d34afe189"