Path manipulation code assumes a Unix filesystem, fails on Windows

[obi1kenobi]

Example failure:

Error: failed to make directory 'C:\Users\runneradmin\.cargo\registry/index\github.com-1ecc6299db9ec823': The system cannot find the path specified.
; class=Os (2)

Encountered in: obi1kenobi/cargo-semver-checks-action#21 (comment)

Appears to correspond to this bit of path manipulation code, which hardcodes a / path separator:

rust-crates-index/src/bare_index.rs

Line 86 in 11fd701

path.push("registry/index");

I also noticed another bit of code that also appears to hardcode a / path separator, though it's a bit tricky to replicate:

rust-crates-index/src/bare_index.rs

Line 17 in 11fd701

let path = base.join(".cargo/config.toml");