fix: restore --no-prebuilt-dart-sdk on mac-arm64 dart_sdk step

Partial revert of #155 for the 3.44.1-rc1 release. With the flag
dropped, the kernel-compiling host steps (const_finder.dart.snapshot,
flutter_patched_sdk dills) resolved the upstream CIPD Dart prebuilt
(dart_revision fc3da898ea1) instead of our fork, stamping kernels with
an SDK hash our VM (0e6acdc83e) refuses to load:
'ConstFinder failure: Can't load Kernel binary: Invalid SDK hash' on
every release-mode build.

The DEPS macos-arm64 GCS prebuilt dep stays (inert with the flag
restored). #155 can return once every kernel-producing step is covered
by a fork prebuilt and the engine build verifies artifact kernel
hashes against the shipped VM.

shorebird/ci/shards/macos.json

@@ -82,7 +82,7 @@
       },
       {
         "type": "gn_ninja",
-        "gn_args": ["--runtime-mode=release", "--mac-cpu=arm64"],
+        "gn_args": ["--runtime-mode=release", "--mac-cpu=arm64", "--no-prebuilt-dart-sdk"],
         "ninja_targets": ["dart_sdk"],
         "out_dir": "host_release_arm64"
       },