Support `cargo install --path ...` using `rust-toolchain`

Problem
If a crate contains a rust-toolchain file, the default toolchain is used if cargo install --path ... is run from outside of the crate's directory. I expected it to use the toolchain specified in rust-toolchain.

Steps

1. rustup default stable
2. cargo new foo
3. echo nightly >foo/rust-toolchain
4. sed -i '1i#![feature(exact_size_is_empty)]' foo/src/main.rs
5. cargo install --path foo

cd foo && cargo install --path . does work.

Notes

Output of rustup --version: rustup 1.21.1 (7832b2ebe 2019-12-20)
Output of rustup show:

Default host: x86_64-pc-windows-msvc
rustup home:  C:\Users\mkraai\.rustup

installed toolchains
--------------------

stable-x86_64-pc-windows-msvc
nightly-x86_64-pc-windows-msvc

active toolchain
----------------

stable-x86_64-pc-windows-msvc (default)
rustc 1.40.0 (73528e339 2019-12-16)

Start by reproducing the listed rustup and cargo install steps, comparing cargo install --path foo with cd foo && cargo install --path .. Trace how the toolchain is selected for an external path; done means the command uses the crate's rust-toolchain and regression coverage preserves the working in-directory case.