running cargo from build dir can't find rustc

[erickt]

cargo is relying on the $PATH to find rustc, but if one runs cargo with x86_64-apple-darwin/stage2/lib/rustc/x86_64-apple-darwin/bin/cargo install tnetstring, cargo will error out with the unhelpful:

info: Installing with git from https://github.com/erickt/rust-tnetstring...
Cloning into '/Users/etryzelaar/Projects/rust/rust/.cargo/work/0UNNtyDTAsQQT2J1'...
remote: Counting objects: 36, done.
remote: Compressing objects: 100% (25/25), done.
remote: Total 36 (delta 14), reused 30 (delta 8)
Unpacking objects: 100% (36/36), done.
error: rustc failed: 1

cargo should detect this and return a proper error message for this. A proper workaround for this is to make sure that $PATH has an absolute path to the binaries set, as in:

export PATH=x86_64-apple-darwin/stage2/lib/rustc/x86_64-apple-darwin/bin:$PATH

It needs to be absolute because cargo does a chdir to the build directory, which would break a relative path.

[brson]

We have a function, std::os::get_exe_path that cargo can use to determine the path to itself, then use to launch the corresponding rustc.