ignore runtime config in hsts check

[btmo]

I was going through https://hexdocs.pm/phoenix/using_ssl.html to set up SSL for a Phoenix service.
According to the documentation:

It is important to note that force_ssl: is a compile time config, so it normally is set in prod.exs, it will not work when set from runtime.exs.

Sobelow currently warns when runtime.exs contains https settings but no force_ssl.
This PR suggests to ignore the runtime.exs file when checking HSTS.

[houllette]

Change makes total sense to me - thanks for the thorough PR!