fix(plugin-legacy): no build.target override on SSR build

[Tal500]

This very simple PR avoid the plugin-legacy overriding (and warning if exists) of the build.target configuration for SSR.

Sadly, in plugin-legacy, the build.target overriding (and warning if exists) happen on SSR build as well.

In addition, some other plugins, like SvelteKit (together with my legacy support PR there - sveltejs/kit#6265), automatically "fixes" the build target for the SSR build, so this will eliminate the warning we get there as a side effect.

---

What is the purpose of this pull request?

• Bug fix

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.