Fix assetsDir in entryFileNames with Vite Env API

[markdalgleish]

Fixes #13430

When future.unstable_viteEnvironmentApi is enabled, we were respecting the value for environments.client.build.assetsDir when configuring build.rollupOptions.output.entryFileNames, but we weren't falling back to the top-level build.assetsDir if it was missing.

This PR updates the logic so that the assetsDir value falls back to build.assetsDir if environments.client.build.assetsDir is not provided, before finally falling back to the default value of "assets".

[changeset-bot]

🦋 Changeset detected

Latest commit: 4704e00

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
@react-router/dev	Patch
@react-router/fs-routes	Patch
@react-router/remix-routes-option-adapter	Patch
create-react-router	Patch
react-router	Patch
react-router-dom	Patch
@react-router/architect	Patch
@react-router/cloudflare	Patch
@react-router/express	Patch
@react-router/node	Patch
@react-router/serve	Patch

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[github-actions]

🤖 Hello there,

We just published version 7.6.0 which includes this pull request. If you'd like to take it for a test run please try it out and let us know what you think!

Thanks!