Fix incorrect 'Ready in' time for next start (#88589)
## What
Fix the incorrect "Ready in" time displayed when running `next start`.
The bug caused the time to show impossibly large values like "Ready in
29474457.7min" instead of the actual startup duration.
## Why
The `NEXT_PRIVATE_START_TIME` environment variable was not being
properly set/propagated when `startServer()` read it. When the variable
was missing, the code defaulted to `0`, causing the calculation
`Date.now() - 0` to equal the entire Unix timestamp (~1.77 trillion
milliseconds ≈ 29 million minutes).
## How
1. Added a fallback in `cli/next-start.ts` to set
`NEXT_PRIVATE_START_TIME` if it's not already set by `bin/next.ts`
2. Updated the calculation in `start-server.ts` to use `0` as the
duration if `startTime` is falsy, preventing the bug
3. Removed unused performance mark code that was leftover
## Test Plan
Run `next start` on a production build and verify the "Ready in" time
shows a reasonable value (e.g., "Ready in 523ms" instead of millions of
minutes).
