EZP-23783: Assetic rules in Apache2 virtual host are not used when prod environment is used by default if ENVIRONMENT variable is omitted

Fixes Assetic rules in Apache2 virtual host when prod environment is used by default if ENVIRONMENT variable is omitted.

With this change the Assetic rewrite rule is only used for environments different from dev, so prod environment will use the Assetic rules without having to declare the ENVIRONMENT variable.