Auto merge of #4634 - kivikakk:cargo-env-prevails, r=matklad

Auto merge of #4634 - kivikakk:cargo-env-prevails, r=matklad

Use argv[0] for cargo_exe so we don't rely on /proc on Linux

This is a proposed solution to #4450. I'm not at all wedded to the idea or the code, though, so feel free to shoot it down with abandon if this isn't something that'd work out or that you like.

In short, we use the existing `CARGO_ENV` (`"CARGO"`) if present, and only if not do we attempt to perform a lookup with `env::current_exe()` ourselves. This means users without access to `current_exe` (such as Linux without `procfs` mounted) can supply the `CARGO` env var themselves for external commands to use.

My concern here is: what if maybe we intentionally switch cargo binaries and didn't intend for this to happen? Could this ever happen outside a test environment? This kind-of-sorta-happened by accident in the test suite, necessitating the explicit removal of `CARGO_ENV` from the subprocess environment, because the actual cargo executing the test suite propagated its own path into the test subprocess!

/cc @alexcrichton as the originator of the idea of `CARGO_ENV`