docs: add note about environment variables with output: 'server'

[allohamora]

Description (required)

I have added a missing note about bundling the environment variables during output: 'server' build, because it's unclear when you read the docs.

Example of the server output:

Code what does it:
https://github.com/withastro/astro/blob/be566b3476260529645e92021957dd0ed502ce4b/packages/astro/src/env/vite-plugin-env.ts#L175

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[florian-lefebvre]

Hi, thanks for opening a PR! Unfortunately I cannot approve it because its content is wrong. Let me explain:

When using output: 'server', process.env variables are bundled (hardcoded) into the server bundle during the npm run build phase

This is not exactly true. The conditions to make this happen are:

• You must use astro:env AFAIK
• You must use a public env var on a server rendered page

That being said, there's a bug there because we include too many things in this serialized object. I'll send a PR to only include the keys we need, thank you!

This means environment variables become part of the compiled server code rather than being read from the environment at runtime. You will need to rebuild to have fresh .env values.

This is only true for public env vars when using astro:env, which is expected.

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Sorry about that but thanks a lot for opening it tho! Allowed to spot a bug