Fix dt serve command as part of the release process

[elliette]

Fixes #9602

With the introduction of perf_witness (which is currently using Dart SDK version 3.11.0-edge), running dt serve fails with:

[serve] running pub get for DDS in the local dart sdk
/Users/elliottbrooks/dev/dart-sdk/sdk/pkg/dds > /Users/elliottbrooks/dev/devtools/tool/flutter-sdk/bin/dart pub get
Resolving dependencies in `/Users/elliottbrooks/dev/dart-sdk/sdk`...
The current Dart SDK version is 3.11.0-285.0.dev.

Because perf_witness requires SDK version ^3.11.0-edge, version solving failed.
ProcessException: Failed with exit code: 1.

Why?

This is because the dt serve command runs dart pub get inside the pkg/dds directory of the engineer's local Dart SDK.

However, the dart executable it uses is from the Flutter version specified in https://github.com/flutter/devtools/blob/b9d7fc1a4119b3d214a77939f9d75b0c0b25d36a/flutter-candidate.txt.

The fix:

We shouldn't be running dart pub get inside Dart SDK packages, instead we should run gclient sync to fetch dependencies.