feat(nuxt): Don't run source maps related code on Nuxt "prepare"

[s1gr1d]

Stumbled upon this while working on something else. The source map related code was always executed on the package manager's install command and as nuxi prepare runs on postinstall. This step is only important for generating the types, so we can skip all source map related things here so this only runs during the actual build.

I also changed the console log a bit so it's more clear what we are doing.

Nuxt docs: https://nuxt.com/docs/4.x/api/commands/prepare

Closes #18937 (added automatically)

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[andreiborza]

Nice!