Make usage of mutable references to libcnb::Env more ergonomic

[edmorley]

In the Python CNB as part of switching to the new struct layer API, I've started using the pattern of having the layer function accept an env: &mut Env argument, to allow threading the libcnb::Env around the buildpack as env vars are added to it and to allow it to be used when running commands.

However, since mutable references are being used, libcnb::Env's existing IntoIterator implementation doesn't work, instead requiring either casting to an immutable reference (ie: .envs(&*env)) or else calling libcnb::Env::iter directly (ie: .envs(env.iter())).

For example:
https://github.com/heroku/buildpacks-python/blob/92c77a653ecead1d331559bc24f0f27fb72bb9d4/src/layers/python.rs#L149
https://github.com/heroku/buildpacks-python/blob/92c77a653ecead1d331559bc24f0f27fb72bb9d4/src/layers/pip_dependencies.rs#L73

We could avoid the need for this by implementing impl<'a> IntoIterator for &'a mut Env.
And for completeness probably the owned impl IntoIterator for Env too.

See also Rust's 3x IntoIterator implementations for HashMap as an example:
https://github.com/rust-lang/rust/blob/53676730146e38e4697b6204c2ee61a9fd6b7e51/library/std/src/collections/hash/map.rs#L2172-L2224