fix: await buildStart before server start

[sapphi-red]

Description

#12624 reverted #12593 partially but it was missing this line.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• 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.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[sapphi-red]

/ecosystem-ci run vite-plugin-svelte

[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
vite-plugin-svelte	❌ failure

[sapphi-red]

Ummm, I thought this would fix the plugin-svelte ecosystem-ci fail.

[patak-dev]

Interesting, we should still merge this PR though.