cna: update `.gitignore` to ignore all `.env` files (#73415)
## Description
Catch-up #61920 and #72260.
According to [Default Environment
Variables](https://nextjs.org/docs/app/building-your-application/configuring/environment-variables#default-environment-variables)
docs, all `.env` files are excluded in `.gitignore` by default,
However, empty templates aren't applied.
Similar PR: #71890
Co-authored-by: Sam Ko <sam@vercel.com>
