Auto merge of #3789 - vojtechkral:cargo_env, r=alexcrichton

Tell subprocesses the path to self in an env variable #3778

I'm just setting the env var on the process itself, letting subprocesses inherit that, as it's easier than setting for each subprocess individually. I'm not entirely sure this is the right spot though.

Also, I should probably document this somewhere - what would be the best place?

diff --cc src/cargo/lib.rs
Simple merge

diff --cc tests/cargotest/lib.rs
Simple merge

diff --cc tests/init.rs
Simple merge