feat: added support for astro:env

[justindra]

Fixes the error during build below by adding support for astro:env

[ERROR] [config] The adapter astro-sst does not currently support the feature "astro:env getSecret". Your project may not build correctly.

Followed instructions from https://docs.astro.build/en/reference/adapter-reference/#envgetsecret

---

@fwang when you get a chance, it'll be great to have this in