Fix is_absolute on WASI
Fix is_absolute on WASI

WASI does not match `cfg(unix)`, but its paths are Unix-like (`/some/path`) and don't have Windows-like prefixes.

Without this change, `is_absolute` for paths like `/some/path` was returning `false`on a WASI target, which is obviously not true and undesirable.
aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust