sentry init doesn't detect env vars when deciding whether to reuse a project

[sentry-junior]

sentry init is not detecting DSNs supplied via environment variables, so it creates a new project instead of reusing the existing one.

• reported by <@U095QF26U23> in Slack while testing sentry init
• confirmed behavior: init should create a project only when there is no DSN in the repo; if there is one, it should reuse it
• current gap: the repo had the DSN as an env var rather than a hardcoded string, and init did not catch it
• this came up while testing a project with a Next.js app plus a supabase/ folder containing edge functions
• related context from the thread: init also currently does not add integrations during setup; that is expected and separate from this bug

Possible repro shape:

• configure a project to provide the DSN via environment variable
• run sentry init
• observe that init creates a new Sentry project instead of reusing the existing one

This seems like DSN detection is currently limited to hardcoded strings / checked-in config and is missing env-based project reuse.