only exit on watch if stdin is TTY

check if stdin is TTY before checking "end" event,
as non-TTY stdin will always invoke end and exit.

found from running "postcss -w" in a docker container (#370)
and running "postcss -w" in lerna or turborepo.

index.js

@@ -61,7 +61,7 @@ let configFile
 if (argv.env) process.env.NODE_ENV = argv.env
 if (argv.config) argv.config = path.resolve(argv.config)
 
-if (argv.watch) {
+if (argv.watch && process.stdin.isTTY) {
   process.stdin.on('end', () => process.exit(0))
   process.stdin.resume()
 }