Suppress browser launch only when BROWSER=none

[CaryLandholt]

Description

As a follow-up to dc6edce, the browser does not open when in
a non-interactive shell (process.stdout.isTTY === false). This will happen when starting the CRA app from within another process, such as running a backend and front-end concurrently in development.

Expected behavior

The browser should open regardless of interactive state; however, users can suppress browser launch when BROWSER=none (introduced in #1247).

Actual behavior

Browser does not open when app is started (via either npm start or yarn start).