Auto merge of #2681 - alexcrichton:cargo-env-home, r=brson

Auto merge of #2681 - alexcrichton:cargo-env-home, r=brson

Canonicalize CARGO_HOME fallback on Windows

This commit ensures that we always return the same fallback value on Windows
regardless of whichever shell we happen to be run from. We do this by removing
the `$HOME` environment variable which `std::env::home_dir` will inspect to
force it to fall back to the system APIs. If the old directory exists then we
favor that one, but otherwise we favor locations like `C:\Users\$user`

Supercedes and closes #2604