Rename `libcnb::Env::new()` to `libcnb::Env::empty()`

[schneems]

It's unclear that Env::new() doesn't inherit env from the current process. It would be clearer if it was Env::empty() instead.

[Malax]

Unsure if I agree with this one given:

• Docs already state: Creates an empty Env struct.
• Personally, I wouldn't have expected it side-effecting and grabbing the current environment
• It's a breaking change

How strongly are you feeling about this @schneems?

[edmorley]

@schneems Thoughts on the above?

The docs are at: https://docs.rs/libcnb/latest/libcnb/struct.Env.html