Symfony version(s) affected: 4.2.8

Description
Dotenv returns error for valid environment variable syntax.

How to reproduce

1. Add variable to .env like FOOBAR="${FOOBAR:=production}"
2. Run ./bin/console cache:clear

Error:
Script cache:clear --no-warmup --env %PUBLIC_DIR% returned with error code 255
Fatal error: Uncaught Symfony\Component\Dotenv\Exception\FormatException: Unclosed braces on variable expansion in "/export/home/dave/Projects/iprd/4.2/iprd-tools/.env" ...${SYMFONY_ENV:=dev}

Bash test of .env file works as expected
$ unset FOOBAR
$ . .env
$ echo $FOOBAR
production