Installation of `rustc` to a `RUSTUP_HOME` specified with an extended-length path fails on Windows.

While trying to set up crater to run on Windows, I encountered the following error:

[2019-02-08T16:37:58Z INFO  crater::run] running `"C:\\Users\\mackendy\\AppData\\Local\\Temp\\.tmpBgcNDX\\rustup-init.exe" "-y" "--no-modify-path" "--default-toolchain" "stable"`
[2019-02-08T16:37:59Z INFO  crater::run] [stderr] info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
[2019-02-08T16:37:59Z INFO  crater::run] [stderr] info: latest update on 2019-01-17, rust version 1.32.0 (9fda7c223 2019-01-16)
[2019-02-08T16:37:59Z INFO  crater::run] [stderr] info: downloading component 'rustc'
[2019-02-08T16:38:02Z INFO  crater::run] [stderr] info: downloading component 'rust-std'
[2019-02-08T16:38:05Z INFO  crater::run] [stderr] info: downloading component 'cargo'
[2019-02-08T16:38:05Z INFO  crater::run] [stderr] info: downloading component 'rust-docs'
[2019-02-08T16:38:06Z INFO  crater::run] [stderr] info: installing component 'rustc'
[2019-02-08T16:38:06Z INFO  crater::run] [stderr] info: rolling back changes
[2019-02-08T16:38:06Z INFO  crater::run] [stderr] error: failed to extract package
[2019-02-08T16:38:06Z INFO  crater::run] [stderr] info: caused by: The filename, directory name, or volume label syntax is incorrect. (os error 123) when creating dir \\?\C:\Users\mackendy\source\crater\work\local\rustup-home\tmp\rbzjn0zcv3mh1cve_dir\rustc/bin
[2019-02-08T16:38:06Z ERROR crater::utils] unable to install rustup

It appears that rustup is trying to create a directory (ending in "/bin") with a forward slash, but perhaps something more subtle is going on.

Version
rustup-init 1.16.0 (beab5ac2b 2018-12-06)

Reproduce the rustup-init installation on Windows with RUSTUP_HOME set to an extended-length path, using the failure log as the starting point. Trace how the rustc package extraction path is constructed, and consider the issue resolved when the stable toolchain installs successfully without the os error 123 failure.