Use getenv() for KIRBY_HOST

[tillschander]

The contents of $_ENV might be empty, even though environment variables are present:
https://stackoverflow.com/questions/3780866/why-is-my-env-empty

For example:

• The php.ini can disallow filling the of $_ENV
• Variables could end up in $_SERVER instead of $_ENV

By using getenv() instead of $_ENV we can fix some of those issues because it is a bit more liberal when it comes to the detection of environment variables.

[bastianallgeier]

Thanks for the PR!