Manifests contain backslashes when built on windows

[Diggsey]

This results in windows targets being uninstallable on linux with the latest nightly.

Paths should be converted to a platform independent format before being written to the manifest: https://github.com/rust-lang/rust-installer/blob/master/src/generator.rs#L120-L124

[cuviper]

Ah, I see. I dealt with that for tar itself (alexcrichton/tar-rs#106), but I neglected to check the manifest.