fix(worker): force rollup to build worker module under watch mode

[fi3ework]

fix #8172 fix #11786

In build watch mode, the transform hook will not be called after the load hook of vite:worker plugin except in the very first build. So returning a timestamp in the watch mode to indicate rollup re-invoke the transform hook, and the load hook returned value will not be used indeed.

Additional context

---

What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing Guidelines.
• [x] Read the Pull Request Guidelines and follow the PR Title Convention.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [ ] Ideally, include relevant tests that fail without this PR but pass with it.