Symfony version(s) affected: 4.2.4

Description
In security.yaml, trying to set the security of a firewall from an env does not work.

How to reproduce

1. Have a .env file as below:

SECURITY_ENABLED=0

1. Now go to config/packages/security.yaml, have it as below:

parameters:
    security_enabled: '%env(bool:SECURITY_ENABLED)%'

security:
    ...
    firewalls:
        secured_area:
            security: '%security_enabled%'
            pattern: ^/

1. Now run the command

 bin/console debug:config SecurityBundle

and you get the bug

In BooleanNode.php line 29:
                                                                                                                     
  Invalid type for path "security.firewalls.secured_area.security". Expected boolean, but got string.  

I have tried the following .env values

SECURITY_ENABLED=
# OR 
SECURITY_ENABLED=0
# OR
SECURITY_ENABLED=false
# OR
SECURITY_ENABLED='false'

None of the above work. The cache clears though, and when accessing a route, the system behaves as if security is turned on. Substituting security: '%security_enabled%' with security: false works correctly and security is disabled.