Add task env detection

[KCErb]

I think it'd be nice to know that a task runner is calling your app code sometimes. So these changes add an env var to the entrypoint tasks.cr and a helper in the env config.

My specific use case is that I have some code I want to run during server startup, but not during db.migrate and friends. This PR will allow me to guard that code like so:

Something.init unless Lucky::Env.task?

[jwoertink]

I like it 👍 I think you'll need to run ameba or the formatter locally so that part passes, but the specs are gonna fail until we get a few other things in.

[jwoertink]

Ok, you'll have to rebase, but then specs will be good 👍